WebComment: display comment submission deadline

* Displays the comment submission deadlinem, when present.

Co-Authored-By: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Merge branch 'master-webcomment-display-deadline' of https://github.com/kasioumis/invenio into prod

Conflicts:
	modules/webcomment/lib/webcomment_config.py
	modules/webcomment/lib/webcomment_templates.py