Merge branch 'fix-broken-mysql-migration' into 'master'

Fix broken migration in MySQL

`keys` is a reserved name in MySQL, so if this migration actually attempted to remove any duplicate keys it would fail.

Closes #19344

See merge request !5005

db/migrate/20160616102642_remove_duplicated_keys.rb

@@ -4,12 +4,12 @@ class RemoveDuplicatedKeys < ActiveRecord::Migration
     select_all("SELECT fingerprint FROM #{quote_table_name(:keys)} GROUP BY fingerprint HAVING COUNT(*) > 1").each do |row|
       fingerprint = connection.quote(row['fingerprint'])
       execute(%Q{
-        DELETE FROM keys
+        DELETE FROM #{quote_table_name(:keys)}
         WHERE fingerprint = #{fingerprint}
         AND id != (
           SELECT id FROM (
             SELECT max(id) AS id
-            FROM keys
+            FROM #{quote_table_name(:keys)}
             WHERE fingerprint = #{fingerprint}
           ) max_ids
         )