[navigation] use add_onload instead of inline javascript href
This way our javascript code isn't thrown out by the html cleaner e.g. when
using the rql rest directive and a table view.
Closes #3501626
[ajax] use a custom tag to handle dynamically loaded js
Using <pre class="script"> makes it trivial for a malicious user to
inject arbitrary javascript into a html or rest text element (because it
looks innocent to the html sanitizer). Using a custom tag we can be
sure that it actually comes from our code and not from untrusted user
data. IE ignores custom tags, though, so we put it in its own namespace.
https://extranet.logilab.fr/1530578