Gantt view time axis: time zone should be scheduler UTC mode

[hjoliver]

See discussion on #1946

Requires exposing UTC mode via the schema.

Not super high priority as most (but not all) of our users go UTC-everything.

[MetRonnie]

Can we close this and consolidate with #1957?

[hjoliver]

This is 1957! Also, it is a bug IMO. If you've asked to see event times etc. in local time, and you don't live in the UK, you don't want to see UTC times - it's a non-trivial translation to do on the fly in your head.

[MetRonnie]

1917 sorry