[WIP] Downstream changes and extract email sender domain

I'm sick of having a private version of Samson. This is an attempt to bring anything from our private version of Samson which should be in the public version and extract a few extra configuration options to ENV variables

### TODO
- [ ] Squash the history on this branch

cc /@zendesk/samson
Thinking initializer, could only run once and save as a constant.
On 24/12/2014 5:01 pm, "Jason Smale" <notifications@github.com> wrote:

> In app/views/sessions/new.html.erb
> <https://github.com/zendesk/samson/pull/220#discussion-diff-22246759>:
>
> > @@ -2,9 +2,6 @@
> >    <h1 class="login-brand">Samson</h1>
> >    <p>Understand what is being deployed.</p>
> >    <div class="login">
> > -    <%= link_to omniauth_path('github'), class: "action github" do %>
>
> @princemaple <https://github.com/princemaple> @steved555
> <https://github.com/steved555> where do you think it would be best to
> define this conditional logic? I'm thinking something like:
>
> def github_auth_enabled?
>   ENV["GITHUB_CLIENT_ID"] && ENV["GITHUB_SECRET"]enddef google_auth_enabled?
>   ENV["GOOGLE_CLIENT_ID"] && ENV["GOOGLE_CLIENT_SECRET"]end
>
> Does Omniauth have any magic to do this.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/zendesk/samson/pull/220/files#r22246759>.
>