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/head_script.tmpl

@@ -38,10 +38,10 @@ If you introduce mistakes in it, Gitea JavaScript code wouldn't run correctly.
 		mermaidMaxSourceCharacters: {{MermaidMaxSourceCharacters}},
 		{{/* this global i18n object should only contain general texts. for specialized texts, it should be provided inside the related modules by: (1) API response (2) HTML data-attribute (3) PageData */}}
 		i18n: {
-			copy_success: '{{.i18n.Tr "copy_success"}}',
-			copy_error: '{{.i18n.Tr "copy_error"}}',
-			error_occurred: '{{.i18n.Tr "error.occurred"}}',
-			network_error: '{{.i18n.Tr "error.network_error"}}',
+			copy_success: '{{.locale.Tr "copy_success"}}',
+			copy_error: '{{.locale.Tr "copy_error"}}',
+			error_occurred: '{{.locale.Tr "error.occurred"}}',
+			network_error: '{{.locale.Tr "error.network_error"}}',
 		},
 	};
 	{{/* in case some pages don't render the pageData, we make sure it is an object to prevent null access */}}