Use UTC as the default timezone instead of the server's timezone.
Rename the filter to `client_tz` instead of `local_tz` to more
accurately reflect it's purpose (once I add the UI for users to
customize their timezone).

Update `request.timezone` to be timezone object (so Pendulum doesn't
need to convert string back to object when rendering).
Wrap RRDtool graph calls in changes to the `TZ` environment variable so
they render in the preferred timezone.
Update graph views to pass timezone to the graph rendering functions.
Fix bug in the fallback for loading timezone from cookie.