Pswadi/login restriction

Added an environment switch to restrict login to emails to company domain.

/cc @zendesk/samson 

### References
 - Jira link:

### Risks
 - None
done. Added test cases for the google login too.


On Wed, Aug 27, 2014 at 10:44 AM, Jason Smale <notifications@github.com>
wrote:

> In app/controllers/sessions_controller.rb:
>
> > @@ -12,10 +12,12 @@ def new
> >    end
> >
> >    def github
> > +    return show_login_restriction unless allowed_to_login
>
> Don't think we should restrict github users. We already verify that they
> are in a certain organisation. Github users don't get a choice of what of
> their registered email addresses they use. I'd prefer this just stays with
> google auth for now.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/zendesk/samson/pull/70/files#r16791840>.
>