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/signup_openid_navbar.tmpl

@@ -1,11 +1,11 @@
 <div class="ui secondary pointing tabular top attached borderless menu stackable new-menu navbar">
 	<div class="new-menu-inner">
 		<a class="{{if .PageIsOpenIDConnect}}active{{end}} item" href="{{AppSubUrl}}/user/openid/connect">
-			{{.i18n.Tr "auth.openid_connect_title"}}
+			{{.locale.Tr "auth.openid_connect_title"}}
 		</a>
 		{{if and .EnableOpenIDSignUp (not .AllowOnlyInternalRegistration)}}
 			<a class="{{if .PageIsOpenIDRegister}}active{{end}} item" href="{{AppSubUrl}}/user/openid/register">
-				{{.i18n.Tr "auth.openid_register_title"}}
+				{{.locale.Tr "auth.openid_register_title"}}
 			</a>
 		{{end}}
 	</div>