Localization refactoring:

* the front-end now use global constants, based on the translation key
* build-client.js use the ESBuild "define" directive to replace these
  globals at compile time, by the english value
* build:client must now be called after build:languages
* moving the loadLoc and loc backend functions in a separate lib

client/common-client-plugin.ts

@@ -22,8 +22,8 @@ async function register ({ peertubeHelpers, registerHook, registerVideoField }:
   })
 
   const [label, description, settings] = await Promise.all([
-    peertubeHelpers.translate('Use chat'),
-    peertubeHelpers.translate('If enabled, there will be a chat next to the video.'),
+    peertubeHelpers.translate(LOC_USE_CHAT),
+    peertubeHelpers.translate(LOC_USE_CHAT_HELP),
     peertubeHelpers.getSettings()
   ])
   const webchatFieldOptions: RegisterClientFormFieldOptions = {