Update metadata file when downloading a solution

[kytrinyx]

If the metadata file has changed (e.g. people have switched from mentored to independent mode, or back) then we'll want to include this data in the solution file.

The download command should be a bit smarter with regards to the metadata file:

• write it if it's not already there
• write it if the uuid matches
• bail with an error if the uuid is different

[Aliciawyse]

@kytrinyx Is this still worth solving? I'd be happy to take up!