feat(i18n): bring the Estonian translation up to date with dev

Translates the keys dev gained since the branch was cut, the help center
above all, and drops the six keys dev no longer has. Registers et
everywhere a language is listed: notification emails, the rate-limit
message, the placeholder check and the wiki. The two tests that counted
the languages by hand now derive the count from SUPPORTED_LANGUAGES.

wiki/Environment-Variables.md

@@ -100,6 +100,7 @@ You can set `DEFAULT_LANGUAGE` to any of the 23 languages TREK ships. The curren
 | `en`    | English            |
 | `de`    | Deutsch            |
 | `es`    | Español            |
+| `et`    | Eesti              |
 | `fr`    | Français           |
 | `hu`    | Magyar             |
 | `nl`    | Nederlands         |