For impersonating access tokens, store impersonatee in the new column

This way, we don't need an index on both columns

crates/collab/src/db/tables/access_token.rs

@@ -7,7 +7,7 @@ pub struct Model {
     #[sea_orm(primary_key)]
     pub id: AccessTokenId,
     pub user_id: UserId,
-    pub impersonator_id: Option<UserId>,
+    pub impersonated_user_id: Option<UserId>,
     pub hash: String,
 }