ci: improve release process, clean up and re-org repo, add automated minification (#143)

* ci: update scripts

* release 2.7.1

* remove tracked stylesheets

* docs: revert stable tag to 2.7.0

* chore: move Plugin source into own dir

* docs: delete info texts

These can now be found in the [project wiki][wiki].

[wiki]: https://github.com/markcheret/footnotes/wiki

* docs: tweak contributing guide

* ci: reflect new directory structure

* chore: update gitignore

* chore: reflect new dir structure

* docs: update documentation

* build(linting): add Husky hooks, Markdown linting, lint all MD files

* fix pre-push command

* fix pre-push command

* build: add stylesheet, JS minification

* ci: add linting steps

* ci: comment out CSS linting step (that's going to be a whole *thing*)

* ci: minify all JS files

* ci: call correct JS file

* chore: lint

* ci: fix PHP linting commands

* chore: increment version constant string

* ci: concat AMP stylesheets

* ci: improve build scripts

* chore: add assets dir

templates/public/amp-reference-container-body.html

@@ -1,39 +0,0 @@
-<!--<?php//for docblock, @see MCI_Footnotes_Template::process_template()
-	/**
-	 * AMP compatible table row default template for uncombined footnotes.
-	 * 
-	 * @since 2.5.11 (draft)
-	 * @since 2.6.0  (release)
-	 *
-	 * - Bugfix: Reference container: apply web semantics to improve readability for assistive technologies, thanks to @derivationfr issue report and code contribution.
-	 *
-	 * @since 2.5.11
-	 *
-	 * @contributor @derivationfr
-	 * @link https://wordpress.org/support/topic/enhance-accesibility-by-using-instead-of/
-	 * @link https://a11y-101.com/development/tables
-	 *
-	 * For assistive technologies to improve accessibility by reading tables in a meaningful manner,
-	 * the tables must be marked up properly using table header elements for header rows and header
-	 * columns as applicable.
-	 * The table caption must also be present for screen readers but may be hidden to sighted users.
-	 */?>
--->
-<tr class="footnotes_plugin_reference_row">
-	<th
-		scope="row"
-		id="footnote_plugin_reference_[[post_id]]_[[container_id]]_[[note_id]]"
-		class="footnote_plugin_index pointer"
-		><[[link-span]]
-			role="button"
-			tabindex="0"
-			class="footnote_plugin_link"
-			[[hard-link]]
-			>[[arrow]][[index]][[terminator]]</[[link-span]]
-		>[[anchor-element]]</th
-	>
-	<td
-		class="footnote_plugin_text"
-		>[[text]]</td
-	>
-</tr>