Improve support for KonText's ucnk_token_auth

[tomachalek]

In short - we should be able to handle refreshing session ID automatically in case the ID expires.
Here is the scenario:

1. WaG client-side sends an AJAX request to KonText with X-Api-Token (= session ID generated using a private token)
2. KonText finds out that the session ID is expired and responses with 401 Unauthorized.
3. WaG client-side calls a special WaG server action refresh_session
4. The refresh_session action will call KonText's /token/authenticate (it means that the WaG server must have a secret token stored in its config) and obtains a new session ID from KonText
5. once WaG client-side gets a proper response, WaG will reload respective tiles (or the whole page in case this would be too cumbersome)