Foreign key relationship from login_token table

[demiankatz]

While validating schemas in #2233, @sambhavp96 noticed that there is no foreign key connecting the login_token table to the user table, even though login_token has a user_id field, and the Doctrine entity in #2233 specifies this relationship.

It looks like the original implementation of the table in #3027 simply omitted the foreign key relationship. A few months later, @EreMaijala added indexes in #4191 to improve performance, so we now have a simple index instead of a foreign key.

I strongly suspect that we'd be better off dropping the user-based index and introducing a foreign key instead; it will have the same benefits as an index while also improving data integrity.

@rajaro, does this make sense to you? Can you think of any reason why we would intentionally avoid using a foreign key here?

If no one objects, @sambhavp96 will likely have time to begin a PR next week to straighten this out (and I'm available to help him as needed).