Tweet button on download page

See https://dev.gitlab.org/gitlab/gitlabhq/issues/1372

"I just installed Gitlab: open source software to collaborate on code. Check it out! https://about.gitlab.com/"

@sytses what do you think of this?

mentioned in merge request !239 (merged)

Status changed to closed by commit 14ec7c

Mentioned in commit 6c1de8b7

Mentioned in merge request !3716 (merged)

Mentioned in commit 6ff6088a

Mentioned in commit dbfb1c0c

Mentioned in commit ce205411

Mentioned in commit 54dfc2ad

Mentioned in commit 043b9f1e