Dont allow git tag rewrite/removal unless you are master

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'tags-protection' into 'master'

Git tags protection

Only masters can remove or modify git tags.

Fixes #1223
Merge branch 'master' of github.com:gitlabhq/gitlabhq