Add mime type for 'aup3' file extension

Add mime type for 'aup3' file extension

The new file extension `.aup3` is not recognized on Linux.

Add a new mime type `application/x-audacity-project+sqlite3` for the
file extension `.aup3` and add this mime type to the supported mime
types of audacity.

Ideally the Audacity team should register two mime types at IATA. These
mime type should probably be named `audio/vnd.audacity.project+sqlite3`
and `audio/vnd.audacity.project+xml`.

Forwarded: https://github.com/audacity/audacity/pull/3713
Signed-off-by: Benjamin Drung <benjamin.drung@canonical.com>
Gbp-Pq: Name Add-mime-type-for-aup3-file-extension.patch