WebComment: more prominent subscription link

* Makes the subscription link more prominent by placing it both at
  the top and at the bottom of the comments, when there are comments.

Co-authored-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Merge branch 'master-webcomment-promote-subscription-link' of https://github.com/kasioumis/invenio into prod

Conflicts:
	modules/webcomment/lib/webcomment_templates.py