manager: Adapt html to stricter rules in angularjs >= 1.8.0

label and textarea 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.

Without this change, the rendering of the modal save form of the manager web interface will "break" halfway through, and only the text "Summary" is visible under the "Saving configuration" header. (This is similar to what is described in #2197 (closed), but with that fixed applied.)

Reference: https://docs.angularjs.org/guide/migration#migrating-from-1-7-to-1-8

(I noticed this as angularjs 1.8.0 just recently hit debian testing. I confirmed that this upgrade was the cause by downgrading to 1.7.9. I have no experience with angular, so I may miss/misunderstand something, but the change did fix the save form of the manager web interface.)