Add UI to enter authorization token when standard auth is not available

[Diadlo]

It's quite common practice to provide authorization with SSO (Single Sign-On) when Mattermost is used in a corporate environment. In this case, the only way to log in is to provide a user field to enter a token obtained from cookies.

Probably in the future we should try to open a browser, wait for user auth and store the token, but let's start with a simple implementation