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/user_mailer/warning.text.erb

@@ -0,0 +1,9 @@
+<%= t "user_mailer.warning.title.#{@warning.action}" %>
+
+===
+
+<% unless @warning.none_action? %>
+<%= t "user_mailer.warning.explanation.#{@warning.action}" %>
+
+<% end %>
+<%= @warning.text %>