Allow editing of first message in discussion thread by its creator

This is similar to #506 but for a discussion thread rather than an issue.

When viewing a discussion thread there's a link to the issue but the issue's description is not displayed, so generally I - and I believe others - copy the same text into both the issue and the first message in the thread. This in itself is far from ideal as highlighted by the 3 (or is it 2?) instances of "There's a lack of cycle parking here" in this combination of issue, thread and comment.

Often these initial posts contain links, as here, and if these links have become stale it would be useful to be able to edit the post. I agree new links should be posted in a new message; the ability to edit the initial message in a discussion thread would allow a link to the new message to be added to the initial message.

Start by reviewing issue #506 and the discussion-thread view described in this issue. Confirm how the first message and its creator are represented; done means the message creator can edit that initial post while later messages remain governed by the existing behavior. The payload names no files or tests.