De-emphasize the unsubscribe link in email notifications

I received a report about multiple cases of users clicking
on the unsubscribe link by accident. The unsubscribe action
does not have a confirmation step so the users get instantly
unsubscribed. I do not want to add a confirmation step, so
instead I'm making a few small changes to hopefully reduce
the chances of accidental clicks:

- The footer text now has a "You are receiving this email because..."
  blurb to separate the footer a little more from the primary
  content
- The unsubscribe link is now a shorter single word: "Unsubscribe",
  making it a smaller click target
- the footer now uses a slightly smaller font than the rest of
  the email message

The people looking for the unsubscribe link should still be able
to easily find it, but hopefully it should now draw less
"accidental attention".

CHANGELOG.md

@@ -9,6 +9,7 @@ All notable changes to this project will be documented in this file.
 - Update CustomHeaderMiddleware to normalize email addresses to lower case (#1074)
 - Add data migration to convert existing user account emails to lower case
 - Update email alerts to mention failure reason (#1069)
+- De-emphasize the unsubscribe link in email notifications
 
 ### Bug Fixes
 - Improve recipient address validation in the smtp listener (#1077)

templates/emails/alert-body-html.html

@@ -156,14 +156,11 @@
 </table>
 {% endif %}
 
-<p style="color: #666666">
+<br>
+<p style="color: #666666; font-size: 90%">
 &mdash;<br>
-{% site_name %}<br>
-<a href="{{ unsub_link }}" target="_blank" style="color: #666666; text-decoration: underline;">
-    {% if check.project.name %}
-    Unsubscribe from "{{ check.project.name }}" notifications
-    {% else %}
-    Unsubscribe
-    {% endif %}
-</a>
+You are receiving this email because you are subscribed to {% site_name %} monitoring
+notifications{% if check.project.name %} for project "{{ check.project.name }}"{% endif %}.<br>
+Don't want to receive these notifications?
+<a href="{{ unsub_link }}" target="_blank" style="color: #666666; text-decoration: underline;">Unsubscribe</a>.
 </p>