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
* use include instead of extend
* add README.rst
* update api doc link in readme.rst
* remove description from __oe__ since it is in README.rst
* add OCA icon and AGPL badges
* add datepicker.js through assets_backend
Luis González
Yanina Aular
Pedro M. Baeza
Stéphane Bidoul
Markus Schneider
Vincent Vinet
Artem Kostyuk
Holger Brunn