Enable 0-min breaks

[dmartinc]

With 0.96, it was possible to add 0 duration breaks to a timetable. Some users (e.g. Atlas) used this to insert "separators" in the timetable or to strongly mark the end of a day.

However in 0.97 the break creation dialog does not allow a duration of 0.

We should also be careful on how these breaks are displayed in the event display page; we should only show the start date (since the end date is the same).

[jbenito]

Same as #438

[jbenito]

Up to now, users where using 0 minutes Breaks in order to add notes or comments in the timetable.
Instead we should provide another entry type like "note" or "comment" with a starting time but without duration.

[pferreir]

The solution is far from optimal, since zero-sized breaks cannot practically not be moved around. This feature will end up causing more problems than benefits.

I would prefer spending some more time in adding an extra layer of "annotations" to the timetable (maybe when we refactor it for drag'n'drop)?

[pferreir]

Ok, since this is needed for 0.98, I will send it back. The current problems are:

• Arrows don't work OK, probably due to duration zero;
• Doing a move that keeps size 0 has absolutely no effect;

Adrian, could you give it a second look?

[Adrian Moennich <jerome.ernst.monnich@…>]

In [4376e4bdec587fe335958c3f61811d53009280d4/indico]:

[IMP] Allow 0-minute breaks

• fixes #231

[Pedro Ferreira <jose.pedro.ferreira@…>]

In [64824a4fe8140c7697d57c5f47d43da97e9964a4/indico]:

[FIX] Some fixes over Adrian's patch

• Zero-sized breaks can now be moved both by time change or using the arrows;
• Fixed the problem that silently closed the edit balloon without error;
• Disabled zero-sized contributions, etc... in the interface;
• Closes #231 for good (I hope);
• Fixed some issues with request listener parameters;