return updated content instead of eventId. refs #435, #444, #461
update after Ajax request should tell how server updated.

current implementation get value from HTML to update pages,
but it should tell how server updated. To implement this,
this diff uses value which is used to build request.
bug fix: initial related event should be updated when ajax succeeded.

previously initial related events in #related-list element
was not updated when Ajax request succeeded. To fix this, we need to
update relatedEventIds in callback function.
Merge pull request #467 from eller86/435-api-spec-fix

435 ModifyAPI spec fix