Frontend refactor: move Vue related code from `index.js` to `components` dir, and...

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 4d78ad9b0e96ca180e0823de17659a2e0814c099.

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