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/base/delete_modal_actions.tmpl

@@ -1,10 +1,10 @@
 <div class="actions">
 	<div class="ui red basic inverted cancel button">
 		{{svg "octicon-x"}}
-		{{.i18n.Tr "modal.no"}}
+		{{.locale.Tr "modal.no"}}
 	</div>
 	<div class="ui green basic inverted ok button">
 		{{svg "octicon-check"}}
-		{{.i18n.Tr "modal.yes"}}
+		{{.locale.Tr "modal.yes"}}
 	</div>
 </div>