Fix MM-207, Create callback for Jitsi server to validated user / host.

[dimashasbi]

This PR works for this Problem Jitsi Issue 207

This PR adds a callback feature when the Jitsi Meeting URL is opened (ex: https://instance.jitsi/room).
Once the URL is open, Jitsi server will send a callback to 'mattermost-server-api' to check user validation.

Process in auth-callback expected 3 conditions.

1. validated then redirect URL to original jitsi space url with valid jwt
2. invalid, then redirect to site url to login
3. error while validating, then redirect url to original jitsi space url but with invalid jwt.

The above 3 conditions have been coded for testing.

Variables for validation are stored in the Plugin Structure.
*note, validation is only based on the room variable in the header only