It looks like the private subnet disconnect button is the only button
we have that does this. There doesn't appear to be a need to use
an ajax submission in this case. However, javascript is still needed
for the confirmation.
Change the javascript to use a normal HTTP form submission if the
method is POST, instead of using an ajax submission. This requires
wrapping the button in an HTML form, so do that in the delete button
template.
Change the private subnet disconnect route to redirect after
successful disconnect, instead of returning 204. Also, simplify the
error handling in the disconnect route, and use
handle_validation_failure.
This allows the specs to use normal Capybara methods for clicking the
button, instead of dropping down to rack-test methods.
Use Disconnect as the label for the disconnect button. Previously, this
just showed a trash icon, which was less accessible to non-visual users,
and even for visual users, odd, because the trash icon is for
deleting objects, and this does a disconnect and not a delete.
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