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/check.py

@@ -0,0 +1,19 @@
+from django.core.checks import Error, register
+
+# the sole purpose of this warning is to prevent people who have
+# django-autocomplete-light installed but not configured to start the app
+@register()
+def dal_check(app_configs, **kwargs):
+    from django.conf import settings
+    from todo.features import HAS_AUTOCOMPLETE
+
+    if not HAS_AUTOCOMPLETE:
+        return
+
+    errors = []
+    missing_apps = {'dal', 'dal_select2'} - set(settings.INSTALLED_APPS)
+    for missing_app in missing_apps:
+        errors.append(
+            Error('{} needs to be in INSTALLED_APPS'.format(missing_app))
+        )
+    return errors