Fix(Workspaces): ensure the presence of a token / server hash in memberships

[pvanliefland]

Changes

• Make sure that access_token (and notebooks_server_hash while we're at it) cannot be blank nor null in WorkspaceMembership
• Removes generate_token method - handle everythin in save() (that's what we were already doing for notebook server hash)
• Added an RunPython on he added migration that calls save() on every membership record without token so that it gets a token

How/what to test

Create a workspace, check in the admin that it has a token and a notebook server hash.

[pvanliefland]

Note: let's put back the blank=True option, and add an admin action for regenerating the access token

[pvanliefland]

@qgerome I skipped the admin action for now, as we are likely to revamp this whole token thing soon. For now you just have to empty the token field in the admin and a new one will be generated