Add "login" index to Users table

[nimmolo]

Without index:

irb(main):001> User.where(login: "Margarida").explain
  User Load (155.7ms)  SELECT `users`.* FROM `users` WHERE `users`.`login` = 'Margarida'
=> 
EXPLAIN SELECT `users`.* FROM `users` WHERE `users`.`login` = 'Margarida'
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | users | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 68627 |     10.0 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set (0.00 sec)

This example is actually straight out of the production log, where it took much longer:

User Load (512.7ms)  SELECT `users`.* FROM `users` WHERE `users`.`login` = 'Margarida' LIMIT 1

---

With index:

irb(main):001> User.where(login: "Margarida").explain
  TRANSACTION (0.1ms)  BEGIN
  User Load (0.4ms)  SELECT `users`.* FROM `users` WHERE `users`.`login` = 'Margarida'
=> 
EXPLAIN SELECT `users`.* FROM `users` WHERE `users`.`login` = 'Margarida'
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | users | NULL       | ref  | login_index   | login_index | 242     | const |    1 |    100.0 | NULL  |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
1 row in set (0.00 sec)

[nimmolo]

Hmm. Can you index on a text column like this? (edit) yes, you can.

[coveralls]

coverage: 94.555%. remained the same
when pulling c59e48e on nimmo-add-target-index-to-comments
into 83b0061 on main.

[mo-nathan]

Looks good.