Disallow New Time Proposal

Summary:
Kolab does not support this feature. E.g. on iOS, when handling
an invitation, "New Time Proposal" option appears, which then does not
work as it requires ResolveRecipients command which we do not support either.
So, if we set DisallowNewTimeProposal=1 iOS will not display this option.

I'm not sure exactly how this new time proposal feature is expected to
work, and whether it would make sense to disable it only for iOS, and e.g. not Outlook.
Kolab Webclient does not support this too.

Reviewers: Syncroton Developers, mollekopf

Reviewed By: Syncroton Developers, mollekopf

Subscribers: Syncroton Developers

Differential Revision: https://git.kolab.org/D4520