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/mail/delivery.py

@@ -0,0 +1,25 @@
+import importlib
+
+def _declare_backend(backend_path):
+    backend_path = backend_path.split('.')
+    backend_module_name = '.'.join(backend_path[:-1])
+    class_name = backend_path[-1]
+
+    def backend(*args, headers={}, from_address=None, **kwargs):
+        def _backend():
+            backend_module = importlib.import_module(backend_module_name)
+            backend = getattr(backend_module, class_name)
+            return backend(*args, **kwargs)
+
+        if from_address is None:
+            raise ValueError("missing from_address")
+
+        _backend.from_address = from_address
+        _backend.headers = headers
+        return _backend
+    return backend
+
+
+smtp_backend = _declare_backend('django.core.mail.backends.smtp.EmailBackend')
+console_backend = _declare_backend('django.core.mail.backends.console.EmailBackend')
+locmem_backend = _declare_backend('django.core.mail.backends.locmem.EmailBackend')