Gallery: update the get_post_galleries method to account for gallery blocks

[glendaviesnz]

Description

Currently the get_post_galleries method will only handle gallery shortcodes. A trac ticket has been open for some time to rectify this, and in the meantime a new version of the gallery block is about to be released which uses innerblocks, so the diffs on this ticket will no longer be sufficient.

This PR adds support for both versions of the gallery block

There was an alternative approach at #1783 which tidies this large method up into a series of smaller methods. This PR makes for a much longer and harder to grok get_post_galleries method, but avoids adding extra methods to global namespace - open to taking either approach depending on which people prefer.

To test:

• Check out this PR to local wp dev env along with the latest version of the gutenberg plugin
• Add a theme that makes use of the get_post_galleries method, eg. Arbutus
• Add several posts using the Gallery post type - add a short code, gallery block, and v2 gallery block (turn on gallery experiment to add a v2 block)
• The the homepage of the site and check that all 3 galleries display as expected

Trac ticket: https://core.trac.wordpress.org/ticket/43826

[ramonjd]

This is testing well for me.

I tried with Classic Block/short code, v1, v2 galleries

Before

After

[glendaviesnz]

Have added initial unit tests - still need to add some tests for the new gallery format

[hellofromtonya]

Closing in preference to PR #1864.