Add Vue linting (#13447)

* Add Vue linting

Turns out the .vue files were not linted at all, so I added that as well
as re-indented the file to 2-space and fixed all reasonable issues that
cam up except one case of a unintended side effect for which I have no
idea how to fix it, so the rule was disabled.

* misc tweaks

* update lockfile

* use overrides to include .vue files

* treat warnings as errors on lint-frontend

* also treat stylelint warnings as errors

* use equal sign syntax

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

package.json

@@ -54,6 +54,7 @@
     "eslint": "7.11.0",
     "eslint-plugin-import": "2.22.1",
     "eslint-plugin-unicorn": "23.0.0",
+    "eslint-plugin-vue": "7.1.0",
     "stylelint": "13.7.2",
     "stylelint-config-standard": "20.0.0",
     "svgo": "1.3.2",