Add email template for sponsors page

sponsors/templates/sponsors/sponsor_petition_email.html

+[PLEASE FILL IN THE TO FIELD ABOVE (keeping admin@scipost.org in cc)]%0D%0A
+%0D%0A
+Dear ...%0D%0A
+%0D%0A
+[PLEASE WRITE A PERSONALIZED MESSAGE]%0D%0A
+%0D%0A
+Here under, you will find basic information about SciPost and how you can support it.%0D%0A
+%0D%0A
+Sincerely,%0D%0A
+[YOUR SIGNATURE]%0D%0A
+%0D%0A
+%0D%0A
+%0D%0A
+SciPost (https://scipost.org) is a top-quality next-generation Genuine Open Access publication portal managed by professional scientists. Its principles, ideals and implementation can be found at https://scipost.org/about and https://scipost.org/FAQ.%0D%0A
+%0D%0A
+SciPost operates on an entirely not-for-profit basis, and charges neither subscription fees nor article processing charges; instead, its activities are financed through a cost-slashing consortial model.%0D%0A
+%0D%0A
+Institutions and organizations that benefit from SciPost’s activities (listed on https://scipost.org/organizations) are cordially invited to become Sponsors. This enables SciPost to perform all of its publication-related activities, maintain its online portal and implement its long-term development plan. Details of the sponsorship scheme and how to join can be found at https://scipost.org/sponsors or by emailing admin@scipost.org.%0D%0A

sponsors/templates/sponsors/sponsors.html

@@ -23,13 +23,13 @@
     </h4>
     <br/>
     <p>
-      Is your organization benefitting from SciPost's activities (check our <a href="{% url 'organizations:organizations' %}">organizations page</a>)? Then consider helping SciPost:
+      Is your organization benefitting from SciPost's activities (check our <a href="{% url 'organizations:organizations' %}">organizations page</a>), and are they not in our list of Sponsors below? Then consider helping SciPost:
     </p>
     <ul>
       <li>
 	<p>
 	  <strong>Are you a scientist?</strong><br/>
-	  Please petition your local librarian/director/... to visit this page and consider sponsoring us.
+	  Please petition your local librarian/director/... to consider sponsoring us. You can use this email <a href="mailto:?subject=Petition to support SciPost&body={% autoescape on %}{% include 'sponsors/sponsor_petition_email.html' %}{% endautoescape %}&cc=admin@scipost.org">template</a>.
 	</p>
       </li>
       <li>