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

app/javascript/controllers/card_hotkeys_controller.js

@@ -79,7 +79,7 @@ export default class extends Controller {
     const selection = this.#selectedCard
     if (!selection) return
 
-    const url = selection.card.dataset.cardAssignToMeUrl
+    const url = selection.card.dataset.cardAssignToSelfUrl
     if (url) {
       event.preventDefault()
       await post(url, { responseKind: "turbo-stream" })