Allow domain blocks that only reject media without silencing or suspending (#4325)

* Allow domain blocks to reject media without silencing or suspending

* Fix typo

* Hide 'Reject media' button when superfluous, instead of disabling it

* Properly hide 'reject media' checkbox on page load if needed

This may happen when resubmitting the domain block form after an error.

* Don't ask whether undoing a media-only block should be retroactive

* Rename :media_only block to :noop

* Display :noop block as None in frontend, allow blocks that do nothing

* Remove 'coding' line auto-added by emacs

app/views/admin/domain_blocks/show.html.haml

@@ -3,13 +3,18 @@
 
 = simple_form_for @domain_block, url: admin_domain_block_path(@domain_block), method: :delete do |f|
 
-  = f.input :retroactive,
-    as: :boolean,
-    wrapper: :with_label,
-    label: t(".retroactive.#{@domain_block.severity}"),
-    hint: t(:affected_accounts,
-      scope: [:admin, :domain_blocks, :show],
-      count: @domain_block.accounts_count)
+  - if (@domain_block.noop?)
+    = f.input :retroactive,
+      as: :hidden,
+      input_html: { :value => "0" }
+  - else
+    = f.input :retroactive,
+      as: :boolean,
+      wrapper: :with_label,
+      label: t(".retroactive.#{@domain_block.severity}"),
+      hint: t(:affected_accounts,
+        scope: [:admin, :domain_blocks, :show],
+        count: @domain_block.accounts_count)
 
   .actions
     = f.button :button, t('.undo'), type: :submit