Self-closing tags are reported by newer versions of the htmllint
Grunt plugin. They also don't make sense in our HTML files
since they are not XHTML-compliant and they run in HTML mode
anyway.
Ref gh-2157
If you generate a Checkboxradio from a checkbox/radio with a label that
contains encoded HTML, e.g. `<em>test</em>` this will work fine
at first. If, however a refresh is triggered on that instance (explicitly or
e.g. by turning it into a `Controlgroup`), the previously escaped HTML will
now be evaluated.
If the label was created based on some user input, this could lead to
unexpected code execution even though the initial output was escaped.
Fixes gh-2101
Closes gh-2102