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/migrations/0008_mail_tracker.py

@@ -0,0 +1,46 @@
+# Generated by Django 2.1.4 on 2018-12-21 14:01
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [("todo", "0007_auto_update_created_date")]
+
+    operations = [
+        migrations.AddField(
+            model_name="comment",
+            name="email_from",
+            field=models.CharField(blank=True, max_length=320, null=True),
+        ),
+        migrations.AddField(
+            model_name="comment",
+            name="email_message_id",
+            field=models.TextField(blank=True, null=True),
+        ),
+        migrations.AlterField(
+            model_name="comment",
+            name="author",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                to=settings.AUTH_USER_MODEL,
+            ),
+        ),
+        migrations.AlterField(
+            model_name="task",
+            name="created_by",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name="todo_created_by",
+                to=settings.AUTH_USER_MODEL,
+            ),
+        ),
+        migrations.AlterUniqueTogether(
+            name="comment", unique_together={("task", "email_message_id")}
+        ),
+    ]