mpd: List browse results before playlists in lsinfo

This makes directories from extensions appear before playlists in the
result of the lsinfo command, which is desirable in clients that does
not sort the result.
stream: metadata_blacklist is optional
config: Lists need to handle missing values
docs: Update changelog with MPD browse imporovements.
mpd: Test order of result from lsinfo
Fix http router so it can handle /{ext_name} same as /{ext_name}/
Merge pull request #756 from dz0ny/fix/static_router_constructor

Fix http router so it can handle /{ext_name} same as /{ext_name}/
http: Use same string interpolation style as elsewhere
http: Group HTTP server tests
http: Redirect from /app to /app/
Merge pull request #757 from jodal/feature/http-apps-without-slash

HTTP: Redirect /app to /app/
mpd: Ignore empty parts in query

This is to fix e.g. that an empty string can't be parsed as a date,
which may be attempted if the query contains 'Date ""'.
mpd: Test that empty query parts are ignored
mpd: Ignore parts in query with only whitespace
web: Remove WebSocket demo page
web: Make pages fill mobile viewports
web: Use font similar to mopidy.com
web: Remove focus on 'HTTP frontend'
web: Strip down CSS
http: Extract helper for setting common response headers