Fix relative URL root path not working for Sidekiq mailers
The current method of setting the relative URL root path does not
appear to work for ActionMailer under Sidekiq. ActionMailer uses
the RAILS_RELATIVE_URL_ROOT
environment to load its default
configuration, and it is impossible to override this setting in
an initializer after the fact.
As a result, e-mail notifications relying on image_tag
or image_url
helpers would fail to include the relative URL root.
Closes #2066 (closed)