Add fixed timeslots for bookable

[Pingviinituutti]

Here's a first version of how the fixed timeslots could work for bookables, according to enhancement #46.

This adds a new field in the admin form. The form is populated with javascript and changes are updated to the text input. If someone knows how to do it in a Django way, feel free to change it.

Features still to do: only suggest the bookable timeslots in the booking form when an user makes a booking.

Also, pls add the hacktoberfest-accepted label to this PR :D

[tlangens]

Instead of a JSON field, I would suggest a separate table with a FK to bookable.
Also instead of end date I would suggest a duration, otherwise there might be issues with DST. Then again I'm not sure what the booking rules for TF's bookables says about DST.

[Pingviinituutti]

Instead of a JSON field, I would suggest a separate table with a FK to bookable.
Also instead of end date I would suggest a duration, otherwise there might be issues with DST. Then again I'm not sure what the booking rules for TF's bookables says about DST.

There! I still kept the end date, because normal bookings also use the end date instead of the duration. This should work if TF's normal booking rules work with DST.

[tlangens]

Suggested change

	if Timeslot.objects.filter(bookable=self).count() > 0: return True
	return False
	return Timeslot.objects.filter(bookable=self).count() > 0