Price options and related events

Make sure to update database fields in the TYPO3 install tool (Database analyzer).

[FEATURE] Added price options #346
[FEATURE] Added related events fields #345
[BUG] Fixed RealUrl Configuration for cancel-Link
[TASK] Added lazy loading to ObjectStorage fields of event #349
[TASK] Add documentation for price option task #347
[TASK] Updated documentation #348