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/javascript/styles/mailer.scss

@@ -426,6 +426,10 @@ h5 {
     background: $success-green;
   }
 
+  &.alert-icon td {
+    background: $error-red;
+  }
+
   img {
     max-width: 32px;
     width: 32px;