Fix actionmailer default hostname etc.

The override setting were being loaded too late because ActionMailer is
configured at the end of _load_config.rb, so move the loading of
overrides into the main _load_config.rb
Disable integrations with issue_trackers etc.

Since the point os using errbit is to not have this data on 3rd-party
servers, it seems sensibel to disable integrations that would forward it
there.
Merge pull request #9 from alphagov/disable_integrations

Disable integrations with issue_trackers etc.
Merge pull request #8 from alphagov/fix_mailer_config

Fix actionmailer default hostname etc.
Timeout sessions after 30 mins inactivity
Merge pull request #11 from alphagov/session_timeout

Timeout sessions after 30 mins inactivity