Improvements to booking rules

[georgmaisser]

• Possibility to turn a rule on and off
• Some possibility to better judge the scope of a rule (this rule will apply to x booking options in y booking instances)
• Possibility to turn off booking rules execution on the level of a single booking option (don't apply rule x/no rules at all)
• On the system rules site, also show all the set rules for each instance (in collapsable, for each instance?)
• Add booking rule to notify before each session date (currently only before course start date)
• Limit non pro users to three booking rules on system level, none on instance level