Rename the Campaigns' column start_time to start_at

This MR enforces the convention on how to name date columns that we want to follow in all our Transformations and Models:

• _at should represent a moment, so that is a timestamp with timezone (or assume it is UTC)
• _date should represent the start of a time unit, such as 2020-01-01 represents the start of 1st of January.

Closes #3 (closed)