DEV: Add rake task to bulk delete posts (#31576)

This PR adds a destroy:posts rake task that can be used to hard-delete a list of posts. Useful for dealing with large amounts of spam that has been soft deleted and needs to go.

Notes:

Works on both non-deleted and soft-deleted posts. (We might want to change this to work on only soft-deleted posts?)
Works exclusively on post IDs. We can't mix topic and post IDs as they might clash, and we have no way of resolving that ambiguity.
Accepts either a rake-style array of IDs or, more conveniently, you can pipe the argument in through STDIN.
Added a confirmation step since it's a fairly destructive operation.

spec/lib/tasks/destroy_rake_spec.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+describe "destroy:posts" do # rubocop:disable RSpec/DescribeClass
+  subject(:task) { subject }
+
+  include_context "in a rake task"
+
+  # No console output in test suite, thanks.
+  before { STDOUT.stubs(:puts) }
+
+  it "accepts a list of post IDs piped through STDIN" do
+    destroy_task = instance_spy(DestroyTask)
+    DestroyTask.stubs(:new).returns(destroy_task)
+
+    STDIN.stubs(:read).returns("1,2,3\n")
+
+    task.invoke
+
+    expect(destroy_task).to have_received(:destroy_posts).with(%w[1 2 3])
+  end
+end