Stream and File extensions in conflict if stream/protocols config includes "file"

[jodal]

From https://discuss.mopidy.com/t/update-to-mopidy-1-1-mopidy-is-not-starting/835:

Aug 11 07:38:43 musik mopidy[24629]: File "/usr/lib/python2.7/dist-packages/mopidy/core/actor.py", line 166, in __init__
Aug 11 07:38:43 musik mopidy[24629]: ) % (scheme, name(b), name(backends_by_scheme[scheme]))
Aug 11 07:38:43 musik mopidy[24629]: AssertionError: Cannot add URI scheme file for FileBackend, it is already handled by StreamBackend

Lots of users have set the stream/protocols config themselves, so they still have file as a protocol that Mopidy-Stream should handle. As of Mopidy 1.1.0, we include a new Mopidy-File backend that claims the file protocol. This causes a conflict between the extensions, and Mopidy shuts down.

As a workaround for this quite common issue, we should filter out file from the values in stream/protocols.

[adamcik]

We might also want to make this conditional on file being enabled.

[jodal]

Fixed by #1254.