Add accessible_to_local_admin flag to AdminAuthenticatedHandler
OrganizationForm: validate activate_by & activated_at as optional
Editable emails (optionally HTML)
Merge branch 'editable-emails'

Conflicts:
	sandy-disaster-recovery/admin_handler/_form_utils.html
	sandy-disaster-recovery/index.yaml
	sandy-disaster-recovery/main.py
	sandy-disaster-recovery/messaging.py
	sandy-disaster-recovery/templates/email/new_organization.to_admins.body.txt
	sandy-disaster-recovery/templates/email/new_organization.to_contacts.body.txt
	sandy-disaster-recovery/templates/email/organization_joins_incident.to_admins.body.txt