Add the ability to export a mutable database

[cldellow]

I've been frustrated a few times by the hassle to get a copy of the mutable DB from Fly to my machine. Let's make it so the "Download DB" link works for mutable databases.

This should perhaps be its own plugin, and we'll just depend on it.

Proposal:

• monkeypatch the DatabaseDownload view
• use VACUUM INTO '/tmp/some-file.db', then stream that back
• tidy up the tmp file somehow - mayber a timer? or maybe we can patch asgi_send_file to delete files that match a certain pattern after sending?
• override the context for database views so allow_download ignores mutability: https://github.com/simonw/datasette/blob/0b4a28691468b5c758df74fa1d72a823813c96bf/datasette/views/database.py#L163-L165

[cldellow]

Fixed in https://github.com/cldellow/datasette-mutable-downloads