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

db/migrate/20181213184704_create_account_warnings.rb

@@ -0,0 +1,12 @@
+class CreateAccountWarnings < ActiveRecord::Migration[5.2]
+  def change
+    create_table :account_warnings do |t|
+      t.belongs_to :account, foreign_key: { on_delete: :nullify }
+      t.belongs_to :target_account, foreign_key: { to_table: 'accounts', on_delete: :cascade }
+      t.integer :action, null: false, default: 0
+      t.text :text, null: false, default: ''
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20181213185533_create_account_warning_presets.rb

@@ -0,0 +1,9 @@
+class CreateAccountWarningPresets < ActiveRecord::Migration[5.2]
+  def change
+    create_table :account_warning_presets do |t|
+      t.text :text, null: false, default: ''
+
+      t.timestamps
+    end
+  end
+end