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Figure out the OAuth token that should be used with this request. We check for this in three
places. In order of priority:
1) From information we cached on the client.
We encrypt the token and cache on the client for performance.
2) From information we have in our persistent state.
We persist the token server-side so we can look it up if necessary.
3) From information the gadget developer tells us to use (a preapproved request token.)
Gadgets can be initialized with preapproved request tokens. If the user tells the service
provider they want to add a gadget to a gadget container site, the service provider can
create a preapproved request token for that site and pass it to the gadget as a user
preference.