Add a system test for joining an account

Reworked the magic link stimulus controller, because the system test
was causing double-submission of the form (because the event was
bubbling up). I think that change simplifies the form and will still
work well for iOS devices.

app/javascript/controllers/magic_link_controller.js

@@ -4,12 +4,18 @@ import { onNextEventLoopTick } from "helpers/timing_helpers"
 export default class extends Controller {
   static targets = [ "input" ]
 
+  submitOnEnter(event) {
+    event.preventDefault()
+    this.submit()
+  }
+
+  submitOnPaste() {
+    onNextEventLoopTick(() => this.submit())
+  }
+
   submit() {
-    onNextEventLoopTick(() => {
-      if (!this.inputTarget.disabled) {
-        this.element.submit()
-        this.inputTarget.disabled = true
-      }
-    })
+    if (this.inputTarget.disabled) return
+    this.element.submit()
+    this.inputTarget.disabled = true
   }
 }