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

src/templates/public/footnote.html

@@ -0,0 +1,23 @@
+<!--<?php//for docblock, @see MCI_Footnotes_Template::process_template()
+	/**
+	 */?>
+-->
+<span
+	class="footnote_referrer"
+	><[[link-span]]
+		role="button"
+		tabindex="0"
+		onclick="footnote_moveToReference_[[post_id]]_[[container_id]]('footnote_plugin_reference_[[post_id]]_[[container_id]]_[[note_id]]');"
+		onkeypress="footnote_moveToReference_[[post_id]]_[[container_id]]('footnote_plugin_reference_[[post_id]]_[[container_id]]_[[note_id]]');"
+		[[hard-link]]
+		><[[sup-span]]
+			id="footnote_plugin_tooltip_[[post_id]]_[[container_id]]_[[note_id]]"
+			class="footnote_plugin_tooltip_text"
+			>[[before]][[index]][[after]]</[[sup-span]]
+		></[[link-span]]
+	>[[anchor-element]]<span
+		id="footnote_plugin_tooltip_text_[[post_id]]_[[container_id]]_[[note_id]]"
+		class="footnote_tooltip"
+		>[[text]]</span
+	></span
+>