[FIX] web_widget_datepicker_options: Apply fixes after migration to v11

This applies some changes to the module after migration, to conform new
guidelines and conventions for version 11.0.

In addition, this applies the following fixes:
- The widged didn't work when the field was a `datetime` due to a typo
  in a JS conditional
- Replaced `Object.assign()` -> `_.assign()` to improve
  browser-compatibility

web_widget_datepicker_options/__init__.py

@@ -1,21 +1 @@
-# -*- encoding: utf-8 -*-
-##############################################################################
-#
-#    OpenERP, Open Source Management Solution
-#    This module copyright (C) 2015 Savoir-faire Linux
-#    (<http://www.savoirfairelinux.com>).
-#
-#    This program is free software: you can redistribute it and/or modify
-#    it under the terms of the GNU Affero General Public License as
-#    published by the Free Software Foundation, either version 3 of the
-#    License, or (at your option) any later version.
-#
-#    This program is distributed in the hope that it will be useful,
-#    but WITHOUT ANY WARRANTY; without even the implied warranty of
-#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-#    GNU Affero General Public License for more details.
-#
-#    You should have received a copy of the GNU Affero General Public License
-#    along with this program.  If not, see <http://www.gnu.org/licenses/>.
-#
-##############################################################################
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).