Add Plugin and Theme check sections

[mattyrob]

This PR adds compatibility sections that check for known incompatible plugins active on an install and also for incompatible theme in use (Twenty Twenty-One)

This change is contingent on an update to the migrate API, for which there is already a PR.

ClassicPress/ClassicPress-APIs#24

[nylen]

I left some specific comments above. More generally, I'm a bit concerned that these checks will offer a false sense of security: since we can't catch all incompatibilities, we probably shouldn't show a green checkmark when the plugin doesn't find anything that's not a known incompatibility. I'm not sure the best way to handle this in the UI though - maybe only show this check if it fails?

[mattyrob]

I have updated the PR as follows:

• 'Pass' icons changes to 'Warn' icons as passing information rather than assuring a smooth migration.
• Fixed trailing white space.
• Added checks for parent / child theme pairings.
• Display names of conflicting plugins on-screen rather than linking to docs / forum for improved UX and easier maintenance.

[ClassyBot]

This pull request has been mentioned on ClassicPress Forums. There might be relevant details there:

https://forums.classicpress.net/t/migration-plugin-work-required/3393/1

[ClassyBot]

This pull request has been mentioned on ClassicPress Forums. There might be relevant details there:

https://forums.classicpress.net/t/migration-plugin-work-required/3393/2

[striebwj]

@mattyrob I think this is good to go now that we got the api update in?

[mattyrob]

@striebwj The API being there means the plugin and theme checks are now operational, provided everyone is happy with the way the information is presented to users when migrating it can be pushed and released. I think all of the comments by @nylen have been addressed.
So, would just need another approval and we can push on.

[striebwj]

Let's push on, merging this and tagging a new release 👍