Enable the Unsupported Block Editor on posts from self-hosted sites

[guarani]

Related: #2358

The missing feature

Enable the Unsupported Block Editor on posts from self-hosted sites. This will allow users to edit posts containing Gutenberg blocks which are supported on their site but not yet supported on Gutenberg Mobile. Currently users can only access the Unsupported Block Editor if the post they're editing is from WordPress.com

Alternatives

There are alternatives to the Unsupported Block Editor, such as using the site's WP Admin on the web to edit posts that contain blocks not supported on mobile.

Additional info

Taken from #3425 (comment):

When the UBE was launched last year [2020], it shipped without support for non-Jetpack self-hosted sites. This was because the editor preference API which the UBE relied upon is a WP.com REST API and is not available in the XML-RPC API that these sites use.

Now that we've removed the dependency on this API – by attempting to load the UBE regardless of whether or not the block editor is available on the site's backend – it makes sense to revisit this and enable self-hosted sites if possible.

[guarani]

👋 In a similar vein to @hypest's comment here regarding an offline experience for the UBE, this issue could also be a good follow-up.

The last we discussed about this was in #3425 (comment), I shared that I didn't see any blockers remaining to enabling the UBE on self-hosted sites.

[guarani]

I think the next steps here would be:

• Enable the UBE on all site types (change could be to return true here on iOS, similar on Android)
• Add test cases to https://github.com/wordpress-mobile/test-cases/blob/trunk/test-cases/gutenberg/unsupported-block-editing.md
• Test
• Deploy!