Django-comments-xtd

* Fixes issue 150, about wrong protocol in the URL when fetching avatar
images from gravatar.

* Fixes issue 145, on inadequate number of SQL queries used by API
entry point "comments-xtd-api-list", available in the URL
"/comments/api/<content-type>/<object-pk>/".
The issue also happened when rendering the comments using tags
get_xtdcomment_tree and render_xtdcomment_tree. It has been fixed
in both cases too.
* Updates the JSON schema of the output retrieved by the API entry
point "comments-xtd-api-list". Thus the version number change.
The flags attribute of each retrieved is now a list of flags instead
of a summary for each the flags: "I like it", "I dislike it",
"suggest removal".

* Fixes issue 138, on unpublishing a single comment with public nested
comments. The fix consists of a new pre_save receiver that will
either publish or unpublish nested comments when a comment changes
its is_public attribute. (thanks to hematinik).

* Fixes issue 144 regarding the size of the JavaScript bundle. The
new JavaScript plugin does not include React and ReactDOM. The two
libraries have to be loaded with an external script.
* Update the dependencies of the JavaScript plugin.

* Fixes issue on the ContentType that happens when sending post
request with empty data. (PR: 137) (thanks to dvorberg).
* Adds German translations, (thanks to dvorberg).

* Adds Django 3.0 compatibility thanks to Sergey Ivanychev (ivanychev).
* Adds Norwegian translations thanks to Yngve Høiseth (yhoiseth).