New feature to add a comment on all slots when adding more slots.

[ptramonti]

Some minimal changes to allow the addition of comments on all slots when adding multiple slots.

[bostelm]

Hello @ptramonti
Thanks for suggesting this. Yes, that's a potentially useful feature, but I think it isn't quite complete: The slotnote editor can contain uploaded files, and these need to be saved, so there's some more work to do (cf. scheduler_editslot_form::save_slot).

Also, I really wouldn't integrate this into the MOODLE_33_STABLE branch, but only into the master branch.