Abstracts: multiple acceptance using track by default

[jbenito]

[From Michel Jouvin]

Based on my recent experience, the ability to accept many abstract in 1 go
has a major drawback : you cannot accept abstract for several tracks at one
or you loose the track information. It'd be great to have one other option
for tracks: use submitted track. With this option, the accepted abstract
will be assigned to the track proposed by the submitter as long as there is
just one. In case the talk has been submitted into several tracks, ideally
the talk will not be accepted (with a message the end of the operation
listing the abstracts not accepted) or less ideally IMO it would be
accepted without being assigned to a track.