scheduler.scheduler_models: Send aborted jobs email to grid maintainers

Instead of sending reports of a job that didn't complete
successfuly, send the emails instead to the grid admins.
This will reduce the noise seen in the results list.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
scheduler.email_manager: Refactor send email code

It's bad to have 2 very similar implementations of a
send email function inside the framework. Let's refactor
the send email function from the email manager code to
use the implementation in base utils.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
base_utils: Add the ability to generate HTML email

Make it possible to send HTML email. Adapt email_manager
to follow suit.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tko.job_serializer: Remove unused imports

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>