Allow disabling rules

[gcampax]

@monicalam says:

"we have to give the choice of making the rule inactive. I want to able to stop it, without having to rewrite the whole rule again."

Some of the code to dynamically enable and disable rules, but currently all rules are automatically enabled. We might need database changes too to persist this information.

[tonyespinoza1]

Specific case needed: stop reminder(s)