Add moderation warnings (#9519)

* Add moderation warnings

Replace individual routes for disabling, silencing, and suspending
a user, as well as the report update route, with a unified account
action controller that allows you to select an action (none,
disable, silence, suspend) as well as whether it should generate an
e-mail notification with optional custom text. That notification,
with the optional custom text, is saved as a warning.

Additionally, there are warning presets you can configure to save
time when performing the above.

* Use Account#local_username_and_domain

app/views/admin/warning_presets/edit.html.haml

@@ -0,0 +1,11 @@
+- content_for :page_title do
+  = t('admin.warning_presets.edit_preset')
+
+= simple_form_for @warning_preset, url: admin_warning_preset_path(@warning_preset) do |f|
+  = render 'shared/error_messages', object: @warning_preset
+
+  .fields-group
+    = f.input :text, wrapper: :with_block_label
+
+  .actions
+    = f.button :button, t('generic.save_changes'), type: :submit