Expose builds feature

Expose builds feature in project settings (as feature).

Enable it by default for a new projects.

I deliberately named it builds instead of CI, because we actualy allow to run tests using infrastructure built-in GitLab. I'm free to change it.

![Screen_Shot_2015-11-09_at_16.42.21](/uploads/a8af0a56fc0498688c0428ff22252d9c/Screen_Shot_2015-11-09_at_16.42.21.png)

If we are ok, I'll add feature tests for it.

/cc @sytses @dzaporozhets 




See merge request !1767