Use photos from source repo where possible
Use photos from source repo where possible

Pulling photos from Flickr every time is fairly slow and makes the build
fairly brittle. Here we introduce a possible source-level cache, and new
photos are fetched only as necessary.


Former-commit-id: d913c3d8679760e3a8fe7823fd7b8404c4898eee
brandur/sorg,brandur/sorg,brandur/sorg,brandur/sorg,brandur/sorg