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/management/commands/mail_worker.py

@@ -0,0 +1,44 @@
+import logging
+import socket
+import sys
+
+from django.core.management.base import BaseCommand
+from django.conf import settings
+
+logger = logging.getLogger(__name__)
+
+
+DEFAULT_IMAP_TIMEOUT = 20
+
+
+class Command(BaseCommand):
+    help = "Starts a mail worker"
+
+    def add_arguments(self, parser):
+        parser.add_argument("--imap_timeout", type=int, default=30)
+        parser.add_argument("worker_name")
+
+    def handle(self, *args, **options):
+        if not hasattr(settings, "TODO_MAIL_TRACKERS"):
+            logger.error("missing TODO_MAIL_TRACKERS setting")
+            sys.exit(1)
+
+        worker_name = options["worker_name"]
+        tracker = settings.TODO_MAIL_TRACKERS.get(worker_name, None)
+        if tracker is None:
+            logger.error(
+                "couldn't find configuration for %r in TODO_MAIL_TRACKERS",
+                worker_name
+            )
+            sys.exit(1)
+
+        # set the default socket timeout (imaplib doesn't enable configuring it)
+        timeout = options["imap_timeout"]
+        if timeout:
+            socket.setdefaulttimeout(timeout)
+
+        # run the mail polling loop
+        producer = tracker["producer"]
+        consumer = tracker["consumer"]
+
+        consumer(producer())