Show "Latest email" column as a relative time (#50)

Exact dates of the last email are usually not super useful when looking
at the commitfest page. This instead starts showing a relative time
since. For the rare cases where the exact date & time is needed, the
tooltip will show the exact date & time. When this gets deployed to prod
a small change also needs to be made to the cfbot, because this breaks
the cfbot its html scraping.

Author: JelteF

pgcommitfest/commitfest/templates/commitfest.html

@@ -124,7 +124,7 @@ <h3>{{p.is_open|yesno:"Active patches,Closed patches"}}</h3>
     <td>{{p.reviewer_names|default:''}}</td>
     <td>{{p.committer|default:''}}</td>
     <td>{{p.num_cfs}}</td>
-    <td style="white-space: nowrap;">{{p.lastmail|date:"Y-m-d"}}<br/>{{p.lastmail|date:"H:i"}}</td>
+    <td style="white-space: nowrap;" title="{{p.lastmail}}">{%if p.lastmail %}{{p.lastmail|timesince}} ago{%endif%}</td>
     {%if user.is_staff%}
      <td style="white-space: nowrap;"><input type="checkbox" class="sender_checkbox" id="send_authors_{{p.id}}">Author<br/><input type="checkbox" class="sender_checkbox" id="send_reviewers_{{p.id}}">Reviewer</td>
     {%endif%}