[MM-45740] Allow users to authenticate using a server that isn't their current server

[devinbinnie]

Summary

NOTE: Depends on this first: https://github.com/mattermost/desktop/pull/2209

When a user tries to use basic authentication where the login request hits a different server (ie. same as custom login flow), it will be rejected since the server URLs don't match.

This PR fixes the auth manager so that it will first check if the server has entered a custom login flow and will allow the user to confirm the login via a permission modal that will show the login URL.

Ticket Link

https://mattermost.atlassian.net/browse/MM-45740

Fixed an issue where users using basic authentication against a different URL than their server URL would be silently rejected.

[mattermod]

Building app in separate branch.

[mattermod]

Successfully building:
https://circleci.com/gh/mattermost/desktop/21459
https://circleci.com/gh/mattermost/desktop/21458
https://circleci.com/gh/mattermost/desktop/21457

[devinbinnie]

I'm going to be doing some work to move external authentication to the browser. Closing this for now.