[web/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.
To make things simpler, we now always use ajax URLs for navigation,
even when we would previously have used regular links.
Closes #3501626
[web/data] fix treeview regression (closes #3526466)
Changeset 68cde7431c2c "[js] remove 3.9 bw compat (where apparently
unused)" removed the use of form.callback from loadxhtml, which treeview
relied on. Update to add a callback on the loadxhtml return value
instead.