TT: save timezone

[katariniss]

I as a developer
Want to detect and save the user's timezone
So that save entries with the appropriate timezone

Description
Need to draw the current timezone from the browser and save it in the state.
When an entry is created or updated, the timezone must be passed to backend.
Display on UI needs to be simple.

Need to check how this new column in database will affect the logic of overlapping time entries.

[katariniss]

The current approach with exclusion constraints won't work, because they can't convert time zones on the fly =>

• Add calculated columns with UTC time; exclusion constraints will work with UTC time.
  OR
• If we can't change the database schema: a function for checking intersections taking into account time zones, where conversion to UTC will take place.

The time zone must be "frozen" when a record is created. Changing the time zone in a profile must not affect historical data.