Allow audio files for use with proofmode

[Hugo22O]

I'd like to use proofmode, but only for audio recordings. My phone records in the .ogg format.

[n8fr8]

• add support for all audio/* mime-types as proof shared via SEND/SHARE intents into ShareProofActivity
• if we can monitor standard audio recording folders, in the way we currently monitor DCIM/Camera photo we should try that as well