Add `AlbumInfo.data_url` property, populate it for Discogs matches and display it when it is non-empty.

Makes it easier for people to find the best Discogs (or other) match
and import it to Music Brainz using a grease monkey or similar script.
`extra_items` and `extra_tracks` on `AlbumInfo` are now lists, sorted by track index and title.
Use turquoise instead of yellow for data source. It's not a warning or penalty, and is easier to distinguish from the partial match string.
Display partial match text before source text. It's a warning with penalty, so probably more important than the data source.
Display disc/media number and title (if available) in track list.

This necessitated a slight tweaking of the line printing code so that
the " * " prefix is not hard coded for all lines. The prefix is now
included in the `lhs` part of lines that actually need it, and this
allows us to alter or omit the prefix for individual lines.
Improved display of missing and unmatched tracks.

Display similar title lines for missing and unmatched tracks, to
separate them from matched tracks. When media numbers or titles are
shown, it looks like missing tracks are grouped under the last media.

With missing and unmatched tracks grouped under a heading now, we don't
need to display "missing track" or "unmatched track" on every line,
making it easier to read the track titles.

Change the prefix from " * " (same as matched tracks) to " ! " for
missing and " ? " for unmatched tracks.

Consistently format track index for both missing and unmatched tracks.
Previously we were omitting the # for missing tracks (compared to
matched tracks), and we were just showing the index (no # or media
number) for unmatched tracks.

Also display track length, if available.
Fix a bug where track number could be half red and half yellow.
Extract disc titles from Discogs' "index tracks".
Improved parsing of Discogs' `position` field.

Some albums have a single disc with positions like I, II, III, IV, etc.
Previously beets thought each track on these albums was a new medium.

Now we assume that if there is no explicit medium index and the ordinal
of an alpha medium does not appear to be sequential (e.g. A, B, C) that
the medium is actually the medium index.
Update change log.