WebAlert to become a BibTask

[skaplun]

Currently the WebAlert engine must be scheduled as a cron job. It should be possible to schedule it as a regular BibTask.

[simko]

Could be done via "fixed time" task concept relatively easily, but let's double check things like maximum number of allowed concurrent processes so that alert engine would surely start everyday at 14:00, say.

Also, this may need to introduce new task constraint facility so that people would not be able to set up sleeping time of one minute by mistake for this task or something. We could perhaps introduce into bibtask configuration a class of tasks that can only run as fixed tasks or only with this-and-this minimum granularity. The aim is to reduce error proneness once this goes out of cron.