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/twofa_scratch.tmpl

@@ -5,18 +5,18 @@
 			<form class="ui form" action="{{.Link}}" method="post">
 				{{.CsrfTokenHtml}}
 				<h3 class="ui top attached header">
-					{{.i18n.Tr "twofa_scratch"}}
+					{{.locale.Tr "twofa_scratch"}}
 				</h3>
 				<div class="ui attached segment">
 					{{template "base/alert" .}}
 					<div class="required inline field">
-						<label for="token">{{.i18n.Tr "auth.scratch_code"}}</label>
+						<label for="token">{{.locale.Tr "auth.scratch_code"}}</label>
 						<input id="token" name="token" type="text" autocomplete="off" autofocus required>
 					</div>
 
 					<div class="inline field">
 						<label></label>
-						<button class="ui green button">{{.i18n.Tr "auth.verify"}}</button>
+						<button class="ui green button">{{.locale.Tr "auth.verify"}}</button>
 					</div>
 				</div>
 			</form>