make scheduled waitlist notifications persistent

[noerw]

a scheduled mail should be sent even after an app restart.

e.g. add a column waitlist_scheduled to each token, which is read on startup and fed into mailer.waitlistSchedule

[SpeckiJ]

Suggestion: Additional DB Table with Token/Timestamp pairs

• Increased performace resulting from less entries to process ? (In theory at least :) )
• Separation between Userdata and internal administrative data
• Implementation of additional administrative data easily possible