Invalid 'In-Reply-To' in the reply button's 'mailto:' link

[Tim Lee]

Below every email in the GNU mailing list archive
(https://lists.gnu.org/archive/html/), there is a 'reply via email to'
button that one can click on to reply to the email.

Unfortunately, the button produces a 'mailto:' link containing an
invalid 'In-Reply-To' value. In particular, the 'In-Reply-To' value
is missing the required angled brackets.

For example, the web interface seems to generate incorrect 'mailto:'
links such as this:

mailto:xyz@example.com?In-Reply-To=20210329xxxxxx&Subject=Example

The correct link should look like this:

mailto:xyz@example.com?In-Reply-To=<20210329xxxxxx>&Subject=Example