Add migration to convert GA timezone to UTC

We are ignoring the timezone from GA requests so previous data needs to
be converted to account for this.

See https://www.pivotaltracker.com/story/show/62779118 for details.

Coupled with https://github.com/alphagov/backdrop-ga-collector/pull/16

## Steps to deploy
- Deploy `backdrop-ga-collector`
- Deploy `backdrop`
- Run migrations

## On preview
- Run `backdrop-ga-collector`
- Verify no duplicate records are present (due to id function changing)