Screenshot file size limits

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.

Start by tracing how screenshots are uploaded and used for cover shots and the Featured carousel. Review the alternative in issue #324, then resolve whether the limit applies only to covers or to all screenshots. Done means the chosen screenshot contexts enforce a byte-size limit without leaving the selection flow unclear.

Written by the indexing model from the issue text.