Show timestamp format tooltips when core supports

Show timestamp format tooltips when core supports

Only change tooltip for away messages when supported by the core.
Restore original tooltip and concatenate together to reuse existing
translations (can be split apart if not a good idea).

Add new feature flag "AwayFormatTimestamp" to indicate when the core
supports parsing timestamp formats in away messages.

This isn't strictly required and if needed can be left out.  It just
might be confusing for the UI to say you can do something.. and it
doesn't work.

Resolves GH-270.