Expiring files and JSON document filters

Added

• Files can expire. An upload can carry an expiry, and expired files are cleaned up through an endpoint of their own. The dashboard gets a button to purge them on demand, shown only to users whose permissions allow it.
• Filter documents with a JSON query. The database browser takes a JSON query and shows only the documents that match, instead of the whole collection.

Fixed

• Setting an expiry on upload and purging expired files both resolved the application a file belongs to by the wrong key, so the record was not found. Both now use the right one.
• The dashboard ran into a parsing error when a user's roles came back in a form it did not expect. That case is handled now.