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/link_account.tmpl

@@ -6,12 +6,12 @@
 			{{if not .AllowOnlyInternalRegistration}}
 				<a class="item {{if not .user_exists}}active{{end}}"
 					data-tab="auth-link-signup-tab">
-					{{.i18n.Tr "auth.oauth_signup_tab"}}
+					{{.locale.Tr "auth.oauth_signup_tab"}}
 				</a>
 			{{end}}
 			<a class="item {{if .user_exists}}active{{end}}"
 				data-tab="auth-link-signin-tab">
-				{{.i18n.Tr "auth.oauth_signin_tab"}}
+				{{.locale.Tr "auth.oauth_signin_tab"}}
 			</a>
 		</div>
 	</div>