Updated tutorial INSTALLED_APPS section (removed contrib.sites)
Fixed #20104 -- Changed VersionDirective in order to avoid ambiguity.

As explained in ticket #20104, the use of versionchanged/versionadded
was confusing.

To solve this ambiguity these directives no longer accept a second
argument but now they only receive the version number (1st arg) and then
a content with the proper comment.
Removed unused imports.
Adapted uses of versionchanged/versionadded to the new form.

Refs #20104.