Use current host as default email address domain

We were using `consul.dev` as default, which might be fine for the
development environment, but it's something that definitely needs to be
changed on production.

Now we're providing a better default setting: using the same domain
which is used to generate URLs in emails. This also makes it easier to
configure new tenants, since the setting will default to whatever host
the new tenant is using.

The `|| "consul.dev"` part might not be needed; I'm keeping it because
I'm not sure production environents would not experience any new issues
if we don't add it, but we might remove it in the future.

db/dev_seeds/settings.rb

@@ -6,7 +6,6 @@ section "Creating Settings" do
     "feature.featured_proposals": "true",
     "feature.map": "true",
     "instagram_handle": "CONSUL",
-    "mailer_from_address": "noreply@consul.dev",
     "mailer_from_name": "CONSUL",
     "meta_description": "Citizen participation tool for an open, "\
                         "transparent and democratic government",