Waitlist, user registration plugin and e-mail attachments

[FEATURE] Frontend plugin for registration management #358
[FEATURE] Added registration waitlist #357
[FEATURE] Added possibility to add e-mail attachments #201
[TASK] Refactored flexform displayCond #335
[DOC] Realurl documentation update for categories #364
[DOC] Added documentation for registration management #363
[DOC] Added documentation for waitlist #360