Add a basic test to cover the comment form rendering.

- move cube files to new folder
- update setup.py, MANIFEST.in, cubicweb_event/__pkginfo__.py, tox.ini

This is better than text/plain, in particular since it preserves list items.

Closes #13810113.

The fix that sanitizes html output from db fields has a little side effect.

use standard form validation instead of custom json controller's methods, support to apply the commment section component on entity not yet created (will be created with the comment)