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

spec/fabricators/account_warning_fabricator.rb

@@ -0,0 +1,5 @@
+Fabricator(:account_warning) do
+  account        nil
+  target_account nil
+  text           "MyText"
+end