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.tmpl

@@ -23,7 +23,7 @@
 							</span>
 						</td>
 						<td>{{FileSize .Size}}</td>
-						<td>{{TimeSince .CreatedUnix.AsTime $.Lang}}</td>
+						<td>{{TimeSince .CreatedUnix.AsTime $.i18n.Lang}}</td>
 						<td class="right aligned">
 							<a class="ui primary show-panel button" href="{{$.Link}}/find?oid={{.Oid}}&size={{.Size}}">{{$.i18n.Tr "repo.settings.lfs_findcommits"}}</a>
 							<button class="ui basic show-modal icon button" data-modal="#delete-{{.Oid}}">

templates/repo/settings/lfs_file_find.tmpl

@@ -37,7 +37,7 @@
 								{{$.i18n.Tr "repo.diff.commit"}}
 								<a class="ui blue sha label" href="{{$.RepoLink}}/commit/{{.SHA}}">{{ShortSha .SHA}}</a>
 							</td>
-							<td>{{TimeSince .When $.Lang}}</td>
+							<td>{{TimeSince .When $.i18n.Lang}}</td>
 						</tr>
 					{{else}}
 						<tr>

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}}