Add synonym index to Names table

[nimmolo]

Name.where(synonym_id: whatever) is called on every obs index and show.

---

without index:

irb(main):001> Name.where(synonym_id: 6869).explain
  Name Load (99.7ms)  SELECT `names`.* FROM `names` WHERE `names`.`synonym_id` = 6869
=> 
EXPLAIN SELECT `names`.* FROM `names` WHERE `names`.`synonym_id` = 6869
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows  | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
|  1 | SIMPLE      | names | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 67680 |     10.0 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set (0.00 sec)

On production, it can be quite slow:

Name Load (494.8ms)  SELECT `names`.* FROM `names` WHERE `names`.`synonym_id` = 6869

---

with index:

irb(main):001> Name.where(synonym_id: 6869).explain
  TRANSACTION (0.1ms)  BEGIN
  Name Load (0.3ms)  SELECT `names`.* FROM `names` WHERE `names`.`synonym_id` = 6869
=> 
EXPLAIN SELECT `names`.* FROM `names` WHERE `names`.`synonym_id` = 6869
+----+-------------+-------+------------+------+---------------+---------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key           | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | names | NULL       | ref  | synonym_index | synonym_index | 5       | const |    2 |    100.0 | NULL  |
+----+-------------+-------+------------+------+---------------+---------------+---------+-------+------+----------+-------+
1 row in set (0.00 sec)

[coveralls]

coverage: 94.517%. remained the same
when pulling 82eceff on nimmo-add-synonym-index-to-names
into f54d931 on main.