Allow form downloads during form entry

[seadowg]

Currently, a user can't fill in an entity form while forms are being updated. We should revise this so that the following rules are true:

• Form entry is not blocked while downloading forms
• Form entry is blocked while processing the form I'm opening or while processing entities that the form uses
• Forms and entities are not perceivably updated during form entry
• Form/entity updates can be downloaded during form entry - a user does not need to be online to then receive the update after form entry
• A form should not be available for form entry without local entities
• Manual downloads/updates can't happen at the same time as background ones

Notes

This would mean a fairly substantial shift in how form updates work. Implementation wise, one possibility would be to split what we now call "download" logic into two distinct parts: "download" and "process":

• download: new forms and form updates (and their media files) are downloaded
• process: downloaded forms are processed - local entities updated, databases/file system updated so that the forms are available

This approach could use a "holding" directory to store new forms/updates that are still to be processed, or it could even work like the automatic form import: forms could be downloaded to forms and then processing could scan for forms not in the DB.

[seadowg]

@lognaturel do you feel like these rules capture everything important?