Frontend refactor: move Vue related code from index.js to components dir, and remove unused codes. (#17301)

* frontend refactor

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

* Update templates/base/head.tmpl

Co-authored-by: delvh <dev.lh@web.de>

* Update docs/content/doc/developers/guidelines-frontend.md

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

* fix typo

* fix typo

* refactor PageData to pageData

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>

* Simply for the visual difference.

Co-authored-by: delvh <dev.lh@web.de>

* Revert "Apply suggestions from code review"

This reverts commit 4d78ad9b0e.

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>

templates/repo/view_file.tmpl

@@ -131,13 +131,3 @@
 		</div>
 	</div>
 </div>
-
-<script>
-function submitDeleteForm() {
-	var message = prompt("{{.i18n.Tr "repo.delete_confirm_message"}}\n\n{{.i18n.Tr "repo.delete_commit_summary"}}", "Delete '{{.TreeName}}'");
-	if (message != null) {
-		$("#delete-message").val(message);
-		$("#delete-file-form").submit()
-	}
-}
-</script>