fix(i18n): every counted string is a plural pair, so one reads singular

"1 seasons", "Synced 1 subscriptions", "1 video(s) were skipped": 21 English
strings carried {{count}} in one plain key and read plural at one. Each is a
_one/_other pair now (Hungarian keeps one form for both), and so is every other
counted string, so the next wording change is a plural too. A contract test
holds the rule for every locale file.

frontend/src/i18n/locales/en/common.json

@@ -14,7 +14,8 @@
   "itemCount_one": "{{formattedCount}} item",
   "itemCount_other": "{{formattedCount}} items",
   "accessRequestsTitle": "Access requests",
-  "accessRequestsMessage": "{{count}} pending — review them on the Users page.",
+  "accessRequestsMessage_one": "{{count}} pending — review it on the Users page.",
+  "accessRequestsMessage_other": "{{count}} pending — review them on the Users page.",
   "accessRequestsReview": "Review",
   "demoTitle": "Demo account",
   "demoSharedNotice": "You're in the shared demo account. Everything you do here — playlists, hidden videos, settings — is communal and may be changed by other demo visitors at the same time.",
@@ -33,7 +34,8 @@
     "lower": "Lower priority",
     "edit": "Click to type a priority (negatives allowed)"
   },
-  "moreRefine": "{{count}} more — refine with search",
+  "moreRefine_one": "{{count}} more — refine with search",
+  "moreRefine_other": "{{count}} more — refine with search",
   "matchMode": "Match",
   "match": {
     "any": "Any",