Implement mail tracker system

* Implement mail tracking

Signed-off-by: Victor "multun" Collod <victor.collod@prologin.org>

* Implement task merging

* Add a mail tracker title format pattern

* Autocomplete task names

* Fix comment display

* Track notification answers

* Add a socket timeout for the mail worker

A mail worker is a long running application. And sometimes, the IMAP server
just hangs for hours for no apparent reason. imaplib doesn't enable setting
a timeout, and setting it globally seems fine.

* Only validate the merge form when submitted

* Redirect to the new form when merging

* Prettier task edit UI

* Make task merging optional

* Test mail tracking

* Update documentation for mail tracking

* Update dependencies

* Add the TODO_COMMENT_CLASSES setting

* Fix dependencies install order

* Remove debug leftovers, improve documentation

* Fail on missing from_address

todo/views/task_autocomplete.py

@@ -0,0 +1,29 @@
+from dal import autocomplete
+from django.contrib.auth.decorators import login_required
+from django.core.exceptions import PermissionDenied
+from django.shortcuts import get_object_or_404
+from django.utils.decorators import method_decorator
+from todo.models import Task
+from todo.utils import user_can_read_task
+
+
+class TaskAutocomplete(autocomplete.Select2QuerySetView):
+    @method_decorator(login_required)
+    def dispatch(self, request, task_id, *args, **kwargs):
+        self.task = get_object_or_404(Task, pk=task_id)
+        if not user_can_read_task(self.task, request.user):
+            raise PermissionDenied
+
+        return super().dispatch(request, task_id, *args, **kwargs)
+
+    def get_queryset(self):
+        # Don't forget to filter out results depending on the visitor !
+        if not self.request.user.is_authenticated:
+            return Task.objects.none()
+
+        qs = Task.objects.filter(task_list=self.task.task_list).exclude(pk=self.task.pk)
+
+        if self.q:
+            qs = qs.filter(title__istartswith=self.q)
+
+        return qs