Using a checkbox wasn't very intuitive because checkboxes are
checked/unchecked when clicked on even if there's an error in the
request. Usually, when checkboxes appear on a form, they don't send any
information to the server unless we click a button to send the form.
So we're using a switch instead of a checkbox, like we did to
enable/disable phases in commit 46d8bc4f0.
Note that, since we've got two switches that match the default
`dom_id(record) .toggle-switch` selector, we need to find a way to
differentiate them. We're adding the `form_class` option for that.
Also note that we're now using a separate action and removing the
JavaScript in the `update` action which assumed that AJAX requests to
this action were always related to updating the `visible_to_valuators`
attribute.
Since this button is replaced by a new element in an AJAX call, nothing
was focused after pressing it.
So we're reusing the code we used to enable/disable budget phases, which
already dealt with this issue.
