Screenshot file size limits

[Warr1024]

Screenshots that are used in certain contexts (e.g. cover shots, Featured carousel) are currently used as-is and can be arbitrarily large or inefficiently-encoded. This can impact total page loading times, make the site seem slow, and consume bandwidth.

The simplest fix would be to limit the byte size of screenshot images, and let the user figure out the necessary encoding options to make an image of the best possible quality within the size constraint. It could be possible to limit this policy only to cover images (but then that would complicate the cover selection UI), or it could just be applied to all screenshots.

An alternative would be #324, but this could be used temporarily while that one is being worked on.

[Warr1024]

Found #324, updated to add reference to that instead.