linkgrp EAD elements need formatting

[seanaery]

Broken out of #898 .

We aren't currently providing formatting the EAD linkgrp element via our render_html_tags helper:
https://www.loc.gov/ead/tglib/elements/linkgrp.html

It seems likely that the contents of such elements should be presented as an unordered list, and that the elements within can be expected to contain linking elements like extref (see #1449) and references to other finding aids, e.g., archref (see #1461).