Bugfixes and code improvements

[BUGFIX] Fix exception in backend module for PID 0 #1003
[BUGFIX] TypeError on calling PageUtility::extendPidListByChildren() #1010 - thanks @web-it-solutions
[BUGFIX] DisplayMode 'current_future' was not implemented within RegistrationRepository #1011 - thanks @web-it-solutions
[BUGFIX] Properly decode email subject fluid string #1005
[TASK] Add plugin icons for content element preview #1012
[TASK] Harden access check event module list action #1004
[TASK] Add typeNum to settings #1002 - thanks @ste101
[TASK] Increase size of link field #1008 - thanks @markussoeth
[TASK] Add more test scenarios for RegistrationRepository #1011
[TASK] Add functional test for CountViewHelper #1015
[TASK] Increase phpstan coverage #1014
[DOCS] Back-quotes are needed for SQL #1000 - thanks @ste101
[DOCS] Fixed usage example for e:category.count viewHelper #1009
[DOCS] Wrong TypoScript path in documentation for migration (plugin.tx_sfeventmgt.settings) #1017 - thanks @web-it-solutions