Update links to ticket tracker.

Using:
`sed -i 's|http://tickets.musicbrainz.org|https://tickets.metabrainz.org|' $(git grep -l tickets.musicbrainz -- './*' ':!*.po')`

README.md

@@ -14,4 +14,4 @@ Binary downloads are available [here](http://picard.musicbrainz.org/downloads/).
 Support and issue reporting
 ---------------------------
 
-Please report all bugs and feature requests in the [MusicBrainz issue tracker](http://tickets.musicbrainz.org/browse/PICARD). If you need support in using Picard please read the [documentation](https://picard.musicbrainz.org/docs/) first and have a look at the [MusicBrainz community forums](https://community.metabrainz.org/c/picard).
+Please report all bugs and feature requests in the [MusicBrainz issue tracker](https://tickets.metabrainz.org/browse/PICARD). If you need support in using Picard please read the [documentation](https://picard.musicbrainz.org/docs/) first and have a look at the [MusicBrainz community forums](https://community.metabrainz.org/c/picard).

picard/coverart/providers/caa.py

@@ -200,7 +200,7 @@ class CoverArtProviderCaa(CoverArtProvider):
     @property
     def _has_suitable_artwork(self):
         # MB web service indicates if CAA has artwork
-        # http://tickets.musicbrainz.org/browse/MBS-4536
+        # https://tickets.metabrainz.org/browse/MBS-4536
         if 'cover_art_archive' not in self.release.children:
             log.debug("No Cover Art Archive information for %s"
                       % self.release.id)

picard/ui/util.py

@@ -47,7 +47,7 @@ class StandardButton(QtGui.QPushButton):
 
 
 # The following code is there to fix
-# http://tickets.musicbrainz.org/browse/PICARD-417
+# https://tickets.metabrainz.org/browse/PICARD-417
 # In some older version of PyQt/sip it's impossible to connect a signal
 # emitting an `int` to a slot expecting a `bool`.
 # By using `enabledSlot` instead we can force python to do the