Setup widget auth popup not handled properly in Element Desktop

[justinbot]

When adding a Slack Workspace in the setup widget, a new window is opened to auth with Slack.
We then wait for the popup to close before refreshing workspaces.

This works in Element Web, but in Element Desktop the popup opens in your web browser so the widget just thinks the popup didn't open:

Authorizing with Slack still works, however you have to close and open the widget to see the new Workspace.
We should implement this in a way that doesn't require interacting with the popup (such as polling workspaces), and fix the popup warning message if possible.