Permalinks work differently for parent emails; cannot link to single email

The permalink to an email produces different results depending on whether the email is the start of a thread or not.

If an email is not at the start of a thread, its permalink will show just that email.
This applies to both stand-alone emails and emails that are part of a thread, but not at the start.

However, once an email has replies, and is therefore part of a thread, its permalink no longer shows the email in isolation. Instead the thread is shown.

It should be possible to link to an individual email using the Permalink.

Start by tracing the permalink handling for emails that have replies and compare it with the handling for standalone or non-parent emails. Add coverage showing that a parent email's permalink opens only that email, then run the relevant test suite to verify both parent and non-parent permalink behavior.

Written by the indexing model from the issue text.