Add the table to track user acknowledgments.
Table structure was described in [ENG-1868](https://linear.app/comm/issue/ENG-1868/create-new-table-to-track-whether-policies-have-been-acknowledged).
There is a code duplication between this and https://phab.comm.dev/D5307 - looks like the idea of `setup-db` was to create an entire database with one query, but if it can be done differently I will extract this query to one function and run the same code in both migration and here.