Reset CSRF token to null when logging out from a server

[sbesson]

With the current state of the extension, the CRSF token acquired on the first invocation to authenticate is not cleared on logout and thus is getting reused by follow-up authentication calls resulting in CSRF errors.

To reproduce the issue

• install and configure the 0.4.1-gs release of the qupath-extension-omero-web, log into an OMERO server
• log into the OMERO instance using a valid username / passwod
• log out from the OMERO instance
• log in again using the same instance & credentials. QuPath should fail to connect
• the OMERO.web logs should include a django.security.csrf WARNING of type log_response():241 Forbidden (CSRF cookie not set.): /api/v0/login/

With these changes included in the extension, the workflow above should be sueccessful

/cc @stick @muhanadz