Make notifs view work for reports

* These are the first small steps for issue 2034 "Reports should send a notification to admins".
* I added a new type of notification "pleroma:report" to the the database manually (a migration will need to be written later)
* I added the new type to the notification_controller
* I made the view return the notification. It doesn't include the report itself (yet)

priv/repo/migrations/20200831152600_add_pleroma_report_to_enum_for_notifications.exs

@@ -0,0 +1,48 @@
+defmodule Pleroma.Repo.Migrations.AddPleromaReportTypeToEnumForNotifications do
+  use Ecto.Migration
+
+  @disable_ddl_transaction true
+
+  def up do
+    """
+    alter type notification_type add value 'pleroma:report'
+    """
+    |> execute()
+  end
+
+  def down do
+    alter table(:notifications) do
+      modify(:type, :string)
+    end
+
+    """
+    delete from notifications where type = 'pleroma:report'
+    """
+    |> execute()
+
+    """
+    drop type if exists notification_type
+    """
+    |> execute()
+
+    """
+    create type notification_type as enum (
+      'follow',
+      'follow_request',
+      'mention',
+      'move',
+      'pleroma:emoji_reaction',
+      'pleroma:chat_mention',
+      'reblog',
+      'favourite'
+    )
+    """
+    |> execute()
+
+    """
+    alter table notifications 
+    alter column type type notification_type using (type::notification_type)
+    """
+    |> execute()
+  end
+end