Notification target contract add tags

[Mirkk]

For contract notifications, there is actually only one Tag contract.time that holds either the notice time or the end time. However it is also sometimes needed to write both dates. This modification adds the contract.noticetime and contract.endtime Tags. The labels used are the ones already in use and thus translated. The tags are shown also on the tags helper page when editing the notification template.

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	none