Fix: Add 'back' button instead of 'cancel' to session request page (#4249)

* Fix: Add 'back' button instead of 'cancel' to session request page

The 'Cancel' button that goes back to the same page was confusing, so change it out for a 'Back' button instead.

Fixes #4007

* Fix: Change absolute target for going back with history.back()

This form can be included from multiple pages, which have differing
navigation for how to land on the form.  Therefore, history.back()
is the single way to navigate back to the previous page.

ietf/secr/templates/includes/sessions_request_form.html

@@ -118,7 +118,7 @@
     <div class="button-group">
         <ul>
             <li><button type="submit" name="submit" value="Save"{% if is_locked %} disabled{% endif %}>Save</button></li>
-            <li><button type="submit" name="submit" value="Cancel">Cancel</button></li>
+            <li><button type="button" onclick="history.back()">Back</button></li>
         </ul>
     </div>
-</form>
+</form>