Refactor i18n, use Locale to provide i18n/translation related functions (#18648)

* remove unnecessary web context data fields, and unify the i18n/translation related functions to `Locale`
* in development, show an error if a translation key is missing
* remove the unnecessary loops `for _, lang := range translation.AllLangs()` for every request, which improves the performance slightly
* use `ctx.Locale.Language()` instead of `ctx.Data["Lang"].(string)`
* add more comments about how the Locale/LangType fields are used

templates/repo/settings/lfs_locks.tmpl

@@ -39,7 +39,7 @@
 									{{$lock.Owner.DisplayName}}
 								</a>
 							</td>
-							<td>{{TimeSince .Created $.Lang}}</td>
+							<td>{{TimeSince .Created $.i18n.Lang}}</td>
 							<td class="right aligned">
 								<form action="{{$.LFSFilesLink}}/locks/{{$lock.ID}}/unlock" method="POST">
 									{{$.CsrfTokenHtml}}