Add support for SLA

[sbesson]

The OMERO sign-up application has been deployed in production onto the OME Demo server

As a RFE, it would be useful to display the Service Level Agreement (SLA), either as a pop-up or as a separate static page linked from the signup page.

[manics]

Since this is an official app we can't embed the SLA. The help text is configurable (though it's missing from the README):
https://github.com/ome/omero-signup/blob/134ec1a9058cd9561a5cd5eb33b13c5f9245d807/omero_signup/signup_settings.py#L39-L40
This could link to an SLA hosted elsewhere.

prod-playbooks/templates/omero-web-config-signup.j2

Line 12 in fd6ebb1

config set -- omero.web.signup.helpmessage 'If you have trouble creating an account please contact {{ omero_server_mail_from | default('[email protected]') }}'

[sbesson]

Transfered from omero-signup as the extensibility mechanism is in place. I think a static page e.g. http://demo.openmicroscopy.org/sla linked from the sign-up page help message would get us most of what we need.