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>

.gitignore

@@ -9,6 +9,8 @@ _test
 
 # IntelliJ
 .idea
+# Goland's output filename can not be set manually
+/go_build_*
 
 # MS VSCode
 .vscode