Add ability to attach an existing photo that is captured outside Mapeo

[gmaclennan]

Currently photos in Mapeo must be taken from within the app. There is no way to attach a photo that was taken with the camera app on the phone, or attach an existing photo on the phone storage.

This has the advantage that it ensures that photos are taken at the same time as the observation (if the user can link another photo, it could be unrelated to the location and time the observation is being recorded). Mapeo should record whether the photo was taken within the app or is imported, and show that to the user in some way.

Desired behaviour

When a user creates an observation, they have the option of selecting an existing photo from the phone photos gallery and attaching it to the observation. The user can distinguish between photos taken within Mapeo and photos attached from the phone photos gallery.