Carry multiple Authentication Tokens

At present when users log into the wen front end they carry a single access token for Google which we use for doc and roster access. In order to incorporate Canvas and Schoology however we will need to carry around multiple tokens and control how those tokens are used on a per-request basis. This means augmenting the roster and document requests to select tokens based upon existing rules dynamically, and having a mechanism to trap errors when the tokens are expired or simply unavailable.

Start by tracing the wen front end’s existing Google access-token flow for roster and document requests. Identify the rules and error paths needed for Canvas and Schoology tokens, with completion defined as per-request token selection and handling expired or unavailable tokens.

Written by the indexing model from the issue text.