This reverses the policy that was set, but incompletely enforced, by
commit 951514dd7d. The self-closing tag
syntax is clearer, more consistent, simpler to parse, compatible with
XML, preferred by Prettier, and (most importantly now) required by
FormatJS.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
We now prevent these variations:
* <hr/>
* <hr />
* <br/>
* <br />
We could enforce similar consistency for other void
tags, if we wished, but these two are particularly
prevalent.