Add sensor / location information for sites.

[cofiem]

Sites need more information about the audio data and the devices that recorded the audio.

This should be done through the notes field.

Add:

• support encoding sensor type
• other key value pairs?

Optional:

• query the notes fields on audio recordings in the sites and aggregate the results (based on Harvesting: Allow inclusion of arbitrary structured meta data baw-workers#25).

NB: on updating this issue, I think it has lost some of its relevance. Especially since I've superseeded some of it in #289. Perhaps it should be closed.