V15: Serverside Media Picker Validation

[nikolajlauridsen]

Adds serverside validation for media picker, also adds a pattern to better support validation for JSON type property editors.

Testing

I've added unit tests, but to test manually:

• Upload media
• Create a document type with a media picker
• Create content based on document type
• Pick some media
• Change media picker configuration to now make the picked media invalid
• Publish - You should now see validation

This way you don't have to mess around with sending raw requests 😄

[AndyButland]

Looks an elegant solution to me. Will do a little manual testing but first leave my comments from reading over the code. Mostly nit-picks but see what you think about more efficiently handing multiple media selections and my comment on the test parameters.

[AndyButland]

Test out fine locally as indicated. Approving with comments, so fine to merge once you have considered the points raised.

[nikolajlauridsen]

Thanks for the review Andy, some good points 👍 I've applied all the points, and added some additional test 👍