Renew auth token

Fixes #164 (closed)

Tasks:

• Store auth token expiry time in model
• Figure out if we need to store credentials in model or if we can store some kind of "token-granting token"
  • There doesn't seem to be a super straightforward way to do this in Keystone.
  • Keystone does have Unscoped tokens, and it looks like there is a expiration setting in the [token] section of keystone.conf, but I don't see an obvious way to set a different, longer timeout for an unscoped token. Even if there were such an option, we can't expect Keystone deployments to be configured in this way.
  • In the near future we will persist application state in local storage, and if the unscoped token expires while the user has the app closed, then we need credentials in order to renew it anyway.
  • So, we store credentials in the model for now.
• Renew auth token "when" it expires
• Prevent a zillion token requests from firing when token expires?
  • I just watched this happen and it didn't break anything, so while not optimal it also doesn't seem to cause a problem right now.
• Test this with a real expiring token