Fix DB error when trying to tag a repository

Steps to reproduce: Go to a project settings, add tags, click on save changes

Observed behavior: Error 500

```
    PG::Error: ERROR:  column "taggings_count" does not exist
        LINE 1: UPDATE "tags" SET "taggings_count" = COALESCE("taggings_coun...
```

Ran `rake acts_as_taggable_on_engine:install:migrations`, removed the first
migration that created the `tags` and `taggings` table, and added the rest.

* Closes #1512
* Closes #1550
* Closes https://github.com/gitlabhq/gitlabhq/issues/6867
* Closes https://github.com/gitlabhq/gitlabhq/issues/9194

See merge request !577