Process embeds for block widgets

[talldanwp]

As noticed in a Gutenberg issue (​https://github.com/WordPress/gutenberg/issues/26180), embeds made using the embed block are not showing on the front end.

From what I understand, the class WP_Embed is responsible for processing embeds in content.

Currently it runs on the_content (post content) and widget_text_content (text widgets), but not on other types of widget:
​https://github.com/WordPress/wordpress-develop/blob/86fc3af215a61f042804e3feb57627976d2bb54d/src/wp-includes/class-wp-embed.php#L32-L40

To solve the bug, I think we'd want it to also run for all block based widgets.

[talldanwp]

I think this is currently blocked awaiting ​https://github.com/WordPress/wordpress-develop/pull/603/files#diff-8d37f4aaee8cc3c4ae950d5fc9bdfada959506a7f5d4c71ba3a546651cc86394 (trac ticket - https://core.trac.wordpress.org/ticket/51506), which adds the WP_Widget_Block class to core from the gutenberg repository.

Once that's in place a filter could be added to that block widget, and then the same technique used to process embeds.

I haven't worked on core embed code before, so if I'm wrong in any of these assumptions, let me know :)

[talldanwp]

Bumping to 5.7 along with other widget related changes.

[hellofromTonya]

This ticket is blocked by #51506 as noted above. 51506 was moved to 5.8 as widgets in core are targeted for 5.8, not 5.7. Moving this ticket with it.

[chaion07]

We reviewed this ticket during a recent [5.8 bug-scrub]​https://wordpress.slack.com/archives/C02RQBWTW/p1622010018432500. Since #51506 isn't a blocker anymore we feel the need of a patch Requesting a much needed attention from contributors. Thank you!

[talldanwp]

Just gave this a test now that block-based widgets have landed in core, and it looks like as well as the code quality improvement in this PR there's also an issue in the customizer where responsive embed don't display in the editor.

[noisysocks]

51566.diff​ adds a new widget_block_content filter which works the same way as widget_text_content.

[noisysocks]

In 51058:

Widget block: Add widget_block_content filter

Adds a new 'widget_block_content' filter to the widget block and hooks
run_shortcode, autoembed, do_blocks, and do_shortcode into it by
default. This is simlar to widget_text_content.

Fixes #51566.
Props talldanwp.

[SergeyBiryukov]

In 51063:

Docs: Correct type for the $widget parameter of the widget_block_content filter.

Follow-up to [51058].

See #51566.