ci: improve linting, GitHub Actions workflows (#149)

* release: release 2.7.2

* fix: urgent 2.7.3 release to fix fatal error

* chore: remove un-needed setup script

* ci: remove steps from pre-push command

* chore: remove un-needed PHP-Commitizen config

* chore: put image files in correct folders

* chore: move GitHub image into `.github/` dir

* fix: classic editor button

* fix: call correct jQuery Tools file in dev env

* docs: replace license with Markdown version

* ci: clean up PHP linting commands

If anyone has noticed me playing musical filepaths with these commands
for a while, it's because I kept getting inconsistent results from the
use of double-globs (i.e., `**`). However, I've finally figured out
that this is because Composer is running these scripts in its own shell,
so the double-glob that works when I run the command manually in my
Terminal doesn't work when Composer runs it in its.

* chore: lint PHP files

* build: update JS linting standards

* chore: lint JS file

* build: add node-sass

* build: add additional stylelint rules

* chore: lint stylesheets with new rulesets

* refactor: move ESLint settings to `package.json`

* chore: move Prettier config to `package.json`

It is not yet possible to move `.prettierignore` to `package.json` too,
but this appears to be on the horizon; see [this Issue][prettier-issue].

[prettier-issue]: https://github.com/prettier/prettier/issues/3460

* fix: move WPML config into Plugin folder

* chore: move Stylelint config into `package.json`

* chore: remove unused `.distignore`

It can always be re-added at a later date if it becomes useful.

* chore: format file

* build: add HTML linting

* fix: add image alt tag

* ci: clean up GitHub Actions workflows

* fix: fix workflow

* fix: fix indentation

* ci: add YAML validation

* chore: make valid

* ci: add YAML validation

* chore: lint code

* ci: change dep install back to original

* chore: lint license

.htmlhintrc

@@ -0,0 +1,22 @@
+{
+	"style-disabled": true,
+	"head-script-disabled": true,
+	"attr-lowercase": true,
+	"attr-no-duplication": true,
+	"attr-no-unnecessary-whitespace": true,
+	"attr-unsafe-chars": true,
+	"attr-value-double-quotes": true,
+	"attr-value-not-empty": false,
+	"alt-require": true,
+	"input-requires-label": true,
+	"href-abs-or-rel": "rel",
+	"tagname-lowercase": true,
+	"empty-tag-not-self-closed": true,
+	"src-not-empty": true,
+	"id-class-ad-disabled": true,
+	"id_class_value": "underline",
+	"id-unique": true,
+	"inline-script-disabled": true,
+	"inline-style-disabled": true,
+	"space-tab-mixed-disabled": true
+}