Add DISABLE_USER_MGMT config value to Flask app

[AndrewEckart]

See ssl-hep/ServiceX_App#73.

[AndrewEckart]

Re: the chart, this would be a good opportunity to reorganize the values a little bit, into something more like:

app:
  auth:
    enabled: true
    disableUserMgmt: false (default)
    globusClientID: *****
    globusClientSecret: *****

I think that the mailgunApiKey, mailgunDomain, and slackSigningSecret should be kept at the app level since those could conceivably be used for other things. I'm not sure about the newSignupWebhook, though, since that only applies when auth and user management are enabled.