[Feature] Add images and galleries to groups.

[herefromrdt]

Is your feature request related to a problem? Please describe.
The current system is very tailored to produced content. It's not very good for event/location based content (like Mardi Gras, WNBR, or the painted ladies of Times Square). You can use tags for this, but it's a little clumsy. Tags are good for grouping content with similar traits, but it would be nice to be able to indicate a more direct relationship between pieces of content, even though they might come from different sources.

Describe the solution you'd like
With the repurposing of movies to the more generic groups, it would be nice if we could use it to group more than scenes. Just the ability to add images and galleries to groups would be helpful. That way you can show that some content is more directly related than they all happen to have blondes in them.

Describe alternatives you've considered
I considered an entire Events section where you can create an event like you would now for a studio or group. Then you could associate all of the content you want to that event. That doesn't seem very scalable though. You can't add an entire section every time you come up with a new way to group content together.
As I said above, I currently use tags for this, which works alright, but I'd rather not use tags for these direct relationships.

Additional context
This also opens up the door for some other features like types of groups (scene, event, location) with more specific Metadata, or group-specific performers for anonymous people who appear in multiple pieces of content, but only within the context of an event or something similar. (Because maybe you don't want to make a performer for them and just make up a name for them that is based on their clothes or something)

[DogmaDragon]

Related #4905