i18n: Introduce plural forms for reservations, emails, messages

In many languages there is more than one plural form of nouns and rules
for choosing the correct one are often far more complex than in English.
Luckily both react-i18next and Weblate provide built-in support for
translating and selecting plural forms in accordance with grammatical
rules of any given language.

In order to enable plural forms `{count: n}` option is added to relevant
`t()` calls. In translations files "_one" and "_other" suffix is added
to English labels such that Weblate can detect which entries represent a
set of plural forms and show appropriate language-specific form on the
translation page. E.g. in Polish there are 2 plural forms and hence 3
resulting suffixes: "_one", "_few", "_many".

Note on transition period: in the absence of expected suffixed variants
react-i18next will use non-suffixed one (if present) so existing
translations will continue to work just fine even if they happen to be
grammatically imperfect. Translators can provide proper plural forms in
once this change is merged and Weblate will then replace non-suffixed
labels with the suffixed ones.

web/public/static/langs/en.json

@@ -225,10 +225,13 @@
   "account_upgrade_dialog_proration_info": "<strong>Proration</strong>: When upgrading between paid plans, the price difference will be <strong>charged immediately</strong>. When downgrading to a lower tier, the balance will be used to pay for future billing periods.",
   "account_upgrade_dialog_reservations_warning_one": "The selected tier allows fewer reserved topics than your current tier. Before changing your tier, <strong>please delete at least one reservation</strong>. You can remove reservations in the <Link>Settings</Link>.",
   "account_upgrade_dialog_reservations_warning_other": "The selected tier allows fewer reserved topics than your current tier. Before changing your tier, <strong>please delete at least {{count}} reservations</strong>. You can remove reservations in the <Link>Settings</Link>.",
-  "account_upgrade_dialog_tier_features_reservations": "{{reservations}} reserved topics",
+  "account_upgrade_dialog_tier_features_reservations_one": "{{reservations}} reserved topic",
+  "account_upgrade_dialog_tier_features_reservations_other": "{{reservations}} reserved topics",
   "account_upgrade_dialog_tier_features_no_reservations": "No reserved topics",
-  "account_upgrade_dialog_tier_features_messages": "{{messages}} daily messages",
-  "account_upgrade_dialog_tier_features_emails": "{{emails}} daily emails",
+  "account_upgrade_dialog_tier_features_messages_one": "{{messages}} daily message",
+  "account_upgrade_dialog_tier_features_messages_other": "{{messages}} daily messages",
+  "account_upgrade_dialog_tier_features_emails_one": "{{emails}} daily email",
+  "account_upgrade_dialog_tier_features_emails_other": "{{emails}} daily emails",
   "account_upgrade_dialog_tier_features_attachment_file_size": "{{filesize}} per file",
   "account_upgrade_dialog_tier_features_attachment_total_size": "{{totalsize}} total storage",
   "account_upgrade_dialog_tier_price_per_month": "month",

web/src/components/UpgradeDialog.js

@@ -297,10 +297,10 @@ const TierCard = (props) => {
                             {monthlyPrice > 0 && <>/ {t("account_upgrade_dialog_tier_price_per_month")}</>}
                         </div>
                         <List dense>
-                            {tier.limits.reservations > 0 && <Feature>{t("account_upgrade_dialog_tier_features_reservations", { reservations: tier.limits.reservations })}</Feature>}
+                            {tier.limits.reservations > 0 && <Feature>{t("account_upgrade_dialog_tier_features_reservations", { reservations: tier.limits.reservations, count: tier.limits.reservations })}</Feature>}
                             {tier.limits.reservations === 0 && <NoFeature>{t("account_upgrade_dialog_tier_features_no_reservations")}</NoFeature>}
-                            <Feature>{t("account_upgrade_dialog_tier_features_messages", { messages: formatNumber(tier.limits.messages) })}</Feature>
-                            <Feature>{t("account_upgrade_dialog_tier_features_emails", { emails: formatNumber(tier.limits.emails) })}</Feature>
+                            <Feature>{t("account_upgrade_dialog_tier_features_messages", { messages: formatNumber(tier.limits.messages), count: tier.limits.messages })}</Feature>
+                            <Feature>{t("account_upgrade_dialog_tier_features_emails", { emails: formatNumber(tier.limits.emails), count: tier.limits.emails })}</Feature>
                             <Feature>{t("account_upgrade_dialog_tier_features_attachment_file_size", { filesize: formatBytes(tier.limits.attachment_file_size, 0) })}</Feature>
                             <Feature>{t("account_upgrade_dialog_tier_features_attachment_total_size", { totalsize: formatBytes(tier.limits.attachment_total_size, 0) })}</Feature>
                         </List>