[DRAFT] MediaMutations: Implement initial model and persistent types.

[scolsen]

Adds basic types for representing media and media mutations separately from other mutations. Media mutations are dependent on submission mutations, but using a more fine-grained representation allows us to both operate on these mutations in a more granular manner and allows us to give more specific information to users.

Note that we do not use these types yet, they are only introduced here.

@gino-m, @sufyanAbbasi - FYI.

[gino-m]

Hi @scolsen, is this ready for review/merge?

[gino-m]

Closing this for now, we can come back to it under the community project.