That way we don't lose the user's input values, we avoid flashing and we
avoid unnecessary rendering.
In the process, fixed an annoying issue where Chrome auto-completes
what it thinks is the username into the "Language Tag" field of the MUC
config form.
Instead we tell Chrome that the MUC JID is the username, thereby also
letting it save the password to to that JID.
This is mainly a testing tool, which makes a mode where all HTML bugs
which could slightly mess up the rendering will trigger proper errors in
the browser.
Fixes#1131.