Allow participation times to deviate from shift times

[felixrindt]

"Optional kann der Helfer eine abweichende Teilnahmezeit angeben."

split off from #25

[felixrindt]

Also check for conflicting confirmed shifts and let users then unoverlap their times.

[jeriox]

Also: the user should be able request a change of his participation time after the event. This should result in a consequence that has to be approved by the responsible person.