Deprecate AMP-specific blocks

[westonruter]

Feature description

We currently have 8 AMP-specific blocks:

We added these originally because we were trying to add AMP features without thinking about long-term whether the blocks would be useful or used. I highly doubt that any of these blocks are being used, aside from AMP Timeago. Also, these blocks are only in Standard mode, further reducing the likelihood they are being used. (This restriction should go away once Bento AMPi is a thing since then they could be freely used on non-AMP pages.)

What's more is that these blocks all say “AMP”. The user shouldn't care these are AMP blocks. They should just be blocks, and they should be split out into separate plugins to only be added if the user even wants them. Functionality should be serving the needs of 80% of users; anything else should be split into other plugins.

So I recommend that we deprecate these blocks, preventing them from even showing up in the list unless one is being used on a page (or on the site?).

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

@jwold FYI

[westonruter]

For the AMP MathML block, we can deprecate in favor of the MathML Block plugin, per adamsilverstein/mathml-block#32.

[westonruter]

QA Passed

When in Standard mode, I added an AMP MathML block to a post, among the other AMP-specific blocks that were provided. After upgrading to 2.1, editing that post shows only the AMP-specific block that I had already added to the post:

And if I start a new post, no such blocks are presented:

So this shows that the blocks are successfully deprecated by not being made available unless they are already being used in the post.