manager: Adapt html to stricter rules in angularjs >= 1.8.0
label elements in XHTML must have a matching closing tag for strict XHTML conformity; when upgrading to angularjs 1.8.0, this became mandatory even when used with angular because of some potential XSS issue.
See https://docs.angularjs.org/guide/migration#migrating-from-1-7-to-1-8
This is similar to !155 (merged), but for widgets I didn't notice had issues at
the time; this issue is seen when e.g. trying to create a new OIDC RP
through the manager, as well as when trying to create a new virtual host.
(Because they use the same modal component.) At this time, I also grepped
for label.*/>
and textarea.*/>
and this is the only locations I found.
This was observed with LLNG 2.0.9+ds-2 in debian bullseye (with libjs-angularjs 1.8.0-1).