Add SCAIP as a Gutenberg block

work-in-progress todo list:

[x] the save function isn't saving; this is because it's returning null which is the proper thing to do for a dynamic block like this one. Can we get it to instead return empty text or an HTML comment?
[x] make sure that the automatic inserter doesn't run with blocks or other already inserted
[x] check that the shortcode is running against the blocks and not aborting because no settings are saved
[x] add an InspectorControls with a help attribute to the edit function's return, like is seen in Pym.js Embeds' settings.
[x] pass alignment classes to the widgets

Changes

Adds a Gutenberg block, for #38
- Choose an Inserted Ad Position sidebar!
- Apply custom classes!
- Use alignment tools! (supports alignwide and alignfull)
- Docs!
Adds docs for inserting shortcodes in aytpical places using custom filters/functions; a continuation of docs for #25's questions about customizability.
Improves helper text on settings page, addressing https://github.com/INN/super-cool-ad-inserter-plugin/issues/37
Improves docs for adding widgets to sidebars.
Improves shortcode docs, adding docs for align and class params added to support the block.
Adds filters to the sidebar before/after widget/title arguments; a continuation of #25's customizability goals.
Misc http -> https upgrades in docs.
Adds support@inn.org as a bug report method.

For #38; so that people can place sidebars carefully on their page, without resorting to shortcodes.

To improve customizability for advanced users.