Fix wrong ARIA attribute in dashboard actions form

Using a `data-toggle` attribute, which we do since commit 07fd5084f,
made Foundation generate an `aria-expanded` attribute to a radio button,
but this attribute can't be present in radio buttons. This makes sense,
since the main purpose of a radio button in a form is to choose an
option, not to show/hide content.

This resulted in the following error when checking the page with Axe:

```
Found 1 accessibility violation:

aria-allowed-attr: Elements must only use supported ARIA attributes
(critical)
https://dequeuniversity.com/rules/axe/4.10/aria-allowed-attr?application=axeAPI
The following 2 nodes violate this rule:

  Selector: #dashboard_action_action_type_proposed_action
  HTML: <input data-toggle="request_to_administrators short_description"
               type="radio" value="proposed_action" checked="checked"
               name="dashboard_action[action_type]"
               id="dashboard_action_action_type_proposed_action"
               aria-expanded="true"
               aria-controls="request_to_administrators">
  Fix all of the following:
    - ARIA attribute is not allowed: aria-expanded="true"

  Selector: #dashboard_action_action_type_resource
  HTML: <input data-toggle="request_to_administrators short_description"
               type="radio" value="resource"
               name="dashboard_action[action_type]"
               id="dashboard_action_action_type_resource"
               aria-expanded="true"
               aria-controls="request_to_administrators">
  Fix all of the following:
    - ARIA attribute is not allowed: aria-expanded="true"
```

So we're using custom JavaScript instead. We're also making the
`short_description` field act as intended; since the changes in commit
07fd5084f it was never shown because it had the `hide` HTML class and it
didn't have a `data-toggler` attribute.

app/helpers/admin/proposal_dashboard_actions_helper.rb

@@ -8,8 +8,4 @@ module Admin::ProposalDashboardActionsHelper
   def default_actions
     %w[polls email poster]
   end
-
-  def css_for_resource(action)
-    "hide" if action == "proposed_action"
-  end
 end