Refactor i18n to locale (#20153)

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

templates/repo/issue/comment_tab.tmpl

@@ -1,6 +1,6 @@
 <div class="ui top tabular menu" data-write="write" data-preview="preview">
-	<a class="active item" data-tab="write">{{.i18n.Tr "write"}}</a>
-	<a class="item" data-tab="preview" data-url="{{.Repository.HTMLURL}}/markdown" data-context="{{.RepoLink}}">{{.i18n.Tr "preview"}}</a>
+	<a class="active item" data-tab="write">{{.locale.Tr "write"}}</a>
+	<a class="item" data-tab="preview" data-url="{{.Repository.HTMLURL}}/markdown" data-context="{{.RepoLink}}">{{.locale.Tr "preview"}}</a>
 </div>
 <div class="field">
 	<div class="ui bottom active tab" data-tab="write">
@@ -9,7 +9,7 @@
 		</textarea>
 	</div>
 	<div class="ui bottom tab markup" data-tab="preview">
-		{{.i18n.Tr "loading"}}
+		{{.locale.Tr "loading"}}
 	</div>
 </div>
 {{if .IsAttachmentEnabled}}