Fix "M" hotkey using stale user from fragment cache

Add a SelfAssignmentsController to resolve the current user at request
time, avoiding the use of an incorrect user id baked into a cached URL.

ref: #2211 originally added the "M" hotkey
ref: https://app.fizzy.do/5986089/cards/3722

test/controllers/cards/self_assignments_controller_test.rb

@@ -0,0 +1,44 @@
+require "test_helper"
+
+class Cards::SelfAssignmentsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in_as :kevin
+  end
+
+  test "create assigns to current user" do
+    card = cards(:layout)
+
+    assert_not card.assigned_to?(users(:kevin))
+
+    post card_self_assignment_path(card), as: :turbo_stream
+    assert_response :success
+    assert_meta_replaced(card)
+    assert card.reload.assigned_to?(users(:kevin))
+  end
+
+  test "create toggles off when already assigned" do
+    card = cards(:logo)
+
+    assert card.assigned_to?(users(:kevin))
+
+    post card_self_assignment_path(card), as: :turbo_stream
+    assert_response :success
+    assert_meta_replaced(card)
+    assert_not card.reload.assigned_to?(users(:kevin))
+  end
+
+  test "create as JSON" do
+    card = cards(:layout)
+
+    assert_not card.assigned_to?(users(:kevin))
+
+    post card_self_assignment_path(card), as: :json
+    assert_response :no_content
+    assert card.reload.assigned_to?(users(:kevin))
+  end
+
+  private
+    def assert_meta_replaced(card)
+      assert_turbo_stream action: :replace, target: dom_id(card, :meta)
+    end
+end