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/mastodon/admin.scss

@@ -542,6 +542,10 @@ a.name-tag,
     border-left-color: lighten($error-red, 12%);
   }
 
+  &.warning {
+    border-left-color: $gold-star;
+  }
+
   &__bubble {
     padding: 16px;
     padding-left: 14px;