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).
Table using only primary key with two columns - (user, policy) and user is in first place on purpose.
A lot of queries will be performed on this table based on the user id. `InnoDB` by default will use [BTREE](https://mariadb.com/kb/en/storage-engine-index-types/) as indexing algorithm which works fine for this approach (keep rows sorted based on user ID and should be decent).
I also did a test with added additional index only for `user` column a perform some queries on table with ~900 rows and ~400 users, measured times were almost equal on both approaches so I think there is no need for another index (might waster some resources) and primary key with pair (user, policy) should be sufficient.