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/user/auth/grant_error.tmpl

@@ -4,11 +4,11 @@
 	<div class="column seven wide">
 		<div class="ui middle centered raised segments">
 			<h1 class="ui top attached header">
-				{{.i18n.Tr "auth.authorization_failed" }}
+				{{.locale.Tr "auth.authorization_failed" }}
 			</h1>
 			<h3 class="ui attached segment">{{.Error.ErrorDescription}}</h3>
 			<div class="ui attached segment">
-				<p>{{.i18n.Tr "auth.authorization_failed_desc"}}</p>
+				<p>{{.locale.Tr "auth.authorization_failed_desc"}}</p>
 			</div>
 		</div>
 	</div>