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/signup_openid_register.tmpl

@@ -4,20 +4,20 @@
 	<div class="ui container">
 				{{template "base/alert" .}}
 				<h4 class="ui top attached header">
-					{{.i18n.Tr "auth.openid_register_title"}}
+					{{.locale.Tr "auth.openid_register_title"}}
 				</h4>
 				<div class="ui attached segment">
 					<p>
-						{{.i18n.Tr "auth.openid_register_desc"}}
+						{{.locale.Tr "auth.openid_register_desc"}}
 					</p>
 					<form class="ui form" action="{{.Link}}" method="post">
 					{{.CsrfTokenHtml}}
 					<div class="required inline field {{if .Err_UserName}}error{{end}}">
-						<label for="user_name">{{.i18n.Tr "username"}}</label>
+						<label for="user_name">{{.locale.Tr "username"}}</label>
 						<input id="user_name" type="text" name="user_name" value="{{.user_name}}" autofocus required>
 					</div>
 					<div class="required inline field {{if .Err_Email}}error{{end}}">
-						<label for="email">{{.i18n.Tr "email"}}</label>
+						<label for="email">{{.locale.Tr "email"}}</label>
 						<input id="email" name="email" type="email" value="{{.email}}" required>
 					</div>
 					{{if and .EnableCaptcha (eq .CaptchaType "image")}}
@@ -26,7 +26,7 @@
 							{{.Captcha.CreateHTML}}
 						</div>
 						<div class="required inline field {{if .Err_Captcha}}error{{end}}">
-							<label for="captcha">{{.i18n.Tr "captcha"}}</label>
+							<label for="captcha">{{.locale.Tr "captcha"}}</label>
 							<input id="captcha" name="captcha" value="{{.captcha}}" autocomplete="off">
 						</div>
 					{{end}}
@@ -46,7 +46,7 @@
 					</div>
 					<div class="inline field">
 						<label></label>
-						<button class="ui green button">{{.i18n.Tr "auth.create_new_account"}}</button>
+						<button class="ui green button">{{.locale.Tr "auth.create_new_account"}}</button>
 					</div>
 					</form>
 				</div>