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

test_settings.py

@@ -29,6 +29,8 @@ INSTALLED_APPS = (
     "django.contrib.sites",
     "django.contrib.staticfiles",
     "todo",
+    "dal",
+    "dal_select2",
 )
 
 ROOT_URLCONF = "base_urls"
@@ -61,3 +63,30 @@ TEMPLATES = [
         },
     }
 ]
+
+LOGGING = {
+    'version': 1,
+    'disable_existing_loggers': False,
+    'handlers': {
+        'console': {
+            'class': 'logging.StreamHandler',
+        },
+    },
+    'loggers': {
+        '': {
+            'handlers': ['console'],
+            'level': 'DEBUG',
+            'propagate': True,
+        },
+        'django': {
+            'handlers': ['console'],
+            'level': 'DEBUG',
+            'propagate': True,
+        },
+        'django.request': {
+            'handlers': ['console'],
+            'level': 'DEBUG',
+            'propagate': True,
+        },
+    },
+}