Make rendered issue and comment fragments as short as possible

For now I made them almost fully correct AFAIS:

no ID conflicts that would show on the same page, e.g. across issue IDs and comment IDs
links seem to go to where we want them to

The only known bug is: cannot link from comment to article

However, in order to achieve this easily we used scopes liberally, and so the fragments are horrendously long.

The ideal fragment setup for both comments and issues would be either:

we don't ever want to show multiple comments/issues from different issues on same page
- issue IDs:
  - regular elements my-header
  - ToC IDs
    the ToC: _toc
    the links: _toc/my-header
- comment IDs:
  - regular elements _comment/1/my-header
  - ToC IDs
    the ToC: _comment/1/_toc
    the links: _comment/1/_toc/my-header
we want to show multiple comments/issues from different issues on same page:
- issue IDs:
  - regular elements _issue/barack-obama/article-topic/1/my-header
  - ToC IDs
    the ToC: _issue/barack-obama/article-topic/1/_toc
    the links: _issue/barack-obama/article-topic/1/_toc/my-header
- comment IDs:
  - regular elements _comment/barack-obama/article-topic/<issue-id>/<comment-id>/my-header
  - ToC IDs
    the ToC: _comment/barack-obama/article-topic/<issue-id>/<comment-id>/_toc
    the links: _comment/barack-obama/article-topic/<issue-id>/<comment-id>/_toc/my-header

 Ancestors

TODO