Allow import and export of metadata

[robbevp]

It would be handy that metadata for albums and tracks can be exported to a file and (partially) imported in another deploy.

This idea still needs a lot of figuring out, but will help people in switching over to the application.

Some open questions:

What part of this will be server-side and whay part client-side?

It would be handy if the RescanRunner could detect these files and automatically use the correct tags (This might also save some editing work if a folder is moved).

On the other hand, clientside has the benefit that import could happen at a later moment and leave the user with a choice in which part of the tags to use.

How can we verify that we are dealing with the same album/artist/tracks?

#9 might be part of the answer, but we'll need some more verification like tracklength, ...