Change the MySQL collation type to utf8_bin to ensure case-sensitive tags are supported

MySQL tables had a collation type of utf8_ci, which makes the name case-insensitive. Previously this migration was run only after indices were built, but it needs to happen before if there is existing data that is case-sensitive. This is an idempotent change, so applying it again in the existing migration (20150425164651) should not change anything.

This is related to !623, but this migration should run before that one.

* Closes #1589
* Closes https://github.com/gitlabhq/gitlabhq/issues/9255

See merge request !625