Improve performance of User.by_login
Performance is improved in two steps: 1. On PostgreSQL an expression index is used for checking lower(email) and lower(username). 2. The check to determine if we're searching for a username or Email is moved to Ruby. Thanks to @haynes for suggesting and writing the initial implementation of this. Moving the check to Ruby makes this method an additional 1.5 times faster compared to doing the check in the SQL query. With performance being improved I've now also tweaked the amount of iterations required by the User.by_login benchmark. This method now runs between 900 and 1000 iterations per second.
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