Add user_id constraint to webpage_activity table

[misaugstad]

The webpage_activity table has a user_id column, but it doesn't have a link to the sidewalk_user table like it should. Should be easy to add. Also need to remember to delete webpage_activity entries with no associated user_id when making the evolution.

[misaugstad]

@dylanbun and when you're doing this, if you happen to see any other places where you think we are missing constraints, let me know! Seems like a good chance to clean up the database a bit :)