RE-style updated labels in discussion tab

(as we changed the "issue-labels" class previously used)
Keep focus when filtering files

Fixed #68
Better tracking of label updates

Because they are a M2M they may be saved after the save of the issue, so
the tracker won't see them.
Corrected now to track m2m changes too, and with a hack of the m2m
descriptor of django to remove removed entries, then add new ones,
instead of clearing all then adding.

Fixed #67
Correct a select2 fofus bug in boostrap modals

See https://github.com/ivaynberg/select2/issues/2152

What we've done: remove the tabindex attribute when the select2 is
created, and set it again when the form is canceled

Fixed #69