Show existing images when editing albums/artists

[chrislo]

At the moment we don't show any existing attached images when editing artists and albums - this is a bit confusing for users as they don't know if the image they've added has actually been attached.

[floehopper]

I've partly addressed this in https://github.com/freerange/music-coop/commit/3aaec284d7d07355620dd646529ba94bcf2287b8.

[floehopper]

I've made a similar change in the artist form in https://github.com/freerange/music-coop/pull/113. However, I've just realised the idea might have been that we'd display the existing image rather than just the filename.

@chrislo Any thoughts?

[floehopper]

I've merged #113, but I'm going to leave this issue open in case we want to display the image rather than just the filename for these form fields.