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/repo/forks.tmpl

@@ -3,7 +3,7 @@
 	{{template "repo/header" .}}
 	<div class="ui container">
 		<h2 class="ui dividing header">
-			{{.i18n.Tr "repo.forks"}}
+			{{.locale.Tr "repo.forks"}}
 		</h2>
 		<div class="ui list">
 			{{range .Forks}}