Problem/Motivation
In fixing #3066802: Ensure that embedded image media items do not display at unreasonable sizes by default, we realized that, once that issue is in, adding a new media type with the "Image"@MediaSource will automatically create a nice, pared-down default display (EntityViewDisplay) for that media type, which just shows the source field.
However, this same nicety doesn't apply to other media types -- if you create, say, a new Remote Video media type, you'll get a bunch of other additional fields (none of which are likely to be as useful to a user as the source field) by default. That kind of sucks.
Proposed resolution
All new media types should, by default, show only the source field in their default display. Every @MediaSource plugin should contain the necessary logic for this.
Remaining tasks
Fix this for all media sources in core and write tests, then commit the patch.
User interface changes
TBD, likely none.
API changes
None.
Data model changes
None.
Release notes snippet
TBD