MVC: Create user_achievements table and UserAchievement model

Now we have the "master" achievements table, we need somewhere to link them to users

We probably need to store:

• user (no index)
• achievement (no index)
• created_by (user)
• revoked (bool, default false, not null)
• revoked_by (user)
• revoked_at (timestamp)

Rather than accepting the default indexes, it might be best to create

• user, revoked
• achievement, revoked

If we consider revocation to be the only way an achievement can be revoked, we could drop the revoked_at column, but it seems a little flakey to me

I'm also a little unsure about storing created_by and revoked_by- it seems like a good idea for audit purposes, but i'm not sure if they should be stored as events so they can be included in activity feeds

I think we should AVOID assuming that user + achievement create a unique key as I envisage achievements which can be awarded multiple times in the future