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/view_content/add_reaction.tmpl

@@ -4,7 +4,7 @@
 		{{svg "octicon-smiley"}}
 	</a>
 	<div class="menu">
-		<div class="header">{{ .ctx.i18n.Tr "repo.pick_reaction"}}</div>
+		<div class="header">{{ .ctx.locale.Tr "repo.pick_reaction"}}</div>
 		<div class="divider"></div>
 		{{range $value := AllowedReactions}}
 			<div class="item reaction" data-content="{{$value}}">{{ReactionToEmoji $value}}</div>