Ability to trigger browser downloads with a content_filename column

[simonw]

Extension on #14. Would add the following header to the response:

Content-Disposition: attachment; filename="filename.jpg"

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Disposition

[simonw]

Needs tests for all four routes:

• content_url that is directly proxied (not transformed)
• content_url that is transformed
• content loaded from database
• content loaded from file on disk

[simonw]

While writing the unit tests for this I figured out SQLite BLOB literals: https://github.com/simonw/til/blob/master/sqlite/blob-literals.md

[simonw]

Documentation: https://github.com/simonw/datasette-media/blob/0.5/README.md#setting-a-download-file-name