Replace literal HTML <a> tags in translated strings

Some translated strings contained literal HTML <a> tags for
hyperlinks. Use "{name|Text}" interpolation instead.
Tiny language fix
MBS-6545: Allow creating tracks with empty number field

When someone tried to create tracks (for a new medium, or additional
tracks for an existing medium) with an empty number field or the
number "0", the number field was automatically set to the track
position on insert. Those values are perfectly legitimate, though.
Merge remote-tracking branch 'chirlu/hyperlinks' into beta

* chirlu/hyperlinks:
  Tiny language fix
  Replace literal HTML <a> tags in translated strings
Merge remote-tracking branch 'chirlu/mbs-6545' into beta

* chirlu/mbs-6545:
  MBS-6545: Allow creating tracks with empty number field
Update mb_server.pot.