Add GitLab.com-only badges

This adds dotcom-only badges, like we already do for the others.

**[BRONZE ONLY]**
**[SILVER ONLY]**
**[GOLD ONLY]**

Review app; http://com-only-badges.178.62.207.141.xip.io/ee/test-badges.html

Closes #356 (closed), extends !244 (merged)

• Once merged, document it https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27546