models: Fix invocation of refresh_tag_counts() for Comments

models: Fix invocation of refresh_tag_counts() for Comments

In Comment.save() and Comment.delete(), we always call
Submission.refresh_tag_counts(), which is an empty stub, rather than
calling Patch.refresh_tag_counts() if the Submission is a Patch.

As such, tag counts are never updated on incoming comments.

Delete Submission.refresh_tag_counts(), as it's useless, and in
Comment.save()/delete(), invoke Patch.refresh_tag_counts() directly when
the submission is a Patch.

Reported-by: David Demelier <markand@malikania.fr>
Fixes: 86172ccc161b ("models: Split Patch into two models")
Closes-bug: #111 ("A/R/T not updated on comments")
Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>