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/settings/webhook/new.tmpl

@@ -5,7 +5,7 @@
 	<div class="ui container">
 		{{template "base/alert" .}}
 		<h4 class="ui top attached header">
-			{{if .PageIsSettingsHooksNew}}{{.i18n.Tr "repo.settings.add_webhook"}}{{else}}{{.i18n.Tr "repo.settings.update_webhook"}}{{end}}
+			{{if .PageIsSettingsHooksNew}}{{.locale.Tr "repo.settings.add_webhook"}}{{else}}{{.locale.Tr "repo.settings.update_webhook"}}{{end}}
 			<div class="ui right">
 				{{if eq .HookType "gitea"}}
 					<img width="26" height="26" src="{{AssetUrlPrefix}}/img/gitea.svg">