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/issue/labels.tmpl

@@ -6,7 +6,7 @@
 			{{template "repo/issue/navbar" .}}
 			{{if and (or .CanWriteIssues .CanWritePulls) (not .Repository.IsArchived)}}
 				<div class="ui right">
-					<div class="ui green new-label button">{{.i18n.Tr "repo.issues.new_label"}}</div>
+					<div class="ui green new-label button">{{.locale.Tr "repo.issues.new_label"}}</div>
 				</div>
 			{{end}}
 		</div>