Major update with TYPO3 8.7 compatibility

Breaking changes:
[!!!][TASK] Restructured flexform for event plugin #418
[!!!][TASK] Move imageWidth and imageHeight to detail section #420

Make sure to use the Extension Update script in order to migrate Flexform changes! Also note, that plugin.tx_sfeventmgt.settings.eventDetails has been renamed to plugin.tx_sfeventmgt.settings.details and TypoScript has to be migrated manually.

See Documentation section "Breaking changes" for more details.

Changes:
[FEATURE] Enable auto confirmation on event basis #398
[FEATURE] Added event data on event registration views #406
[FEATURE] Added error handling for event detail page #407
[FEATURE] Add preview for Plugins #421
[FEATURE] Added support for year/month/day to overwriteDemand #427
[BUGFIX] Fixed link in rss view #390
[BUGFIX] Fix fluid template path for backend module #386
[TASK] Use TYPO3 CSS classes for backend module #425
[TASK] Catch property mapper exceptions for detail view #426
[TASK] Disable overrideDemand in plugin by default
[TASK] Replace event.enableCancel in confirmation email #362
[TASK] Deny cancellation of registration after event started #361
[TASK] Missing field origuid #377
[TASK] Added ACL for event and priceoption #156
[TASK] Move "Create new" button for event IRRE element to bottom #396
[TASK] Added functional tests for orderFieldAllowed #410
[TASK] Changed dates to object in e-mail templates #415
[TASK] Make Update-Script V8 compatible #417
[TASK] Added "gender" to prefillFields and configured field prefilling
[TASK] Remove legacy code for TYPO3 6.2
[TASK] Move ICalendar layout to template
[TASK] TYPO3 7.6 and 8.7 TCA Migrations