UX: Inform users they need to setup a media manager

[DirtyF]

Given you didn't set up a media manager yet
When you hit the image button in the WYSIWYG editor
Then you should be informed that you need to add a media manager and how to do it

Demo: https://www.loom.com/share/1a32467448e14dd1be80ca8e7df392d7

Given you didn't set up a media manager yet
When you hit the image placeholder in an image field
Then you should be informed that you need to add a media manager and how to do it

Currently, nothing happens when you hit image fields in TinaCMS UI when you didn't define a media manager, some user feedback to guide first-time users would be nice.