Create OAuth token management subsystem

[jamesvandyne]

Problem As Tanzawa increases the number of integrations with external services, there will be an increasing amount of similar OAuth glue-code.

Success When all OAuth integrations can use the same OAuth domain/data layer for managing the integrations.

Timing At the time of writing this issue, the Strava integration is still under development. When development starts on the 3rd OAuth based service, this issue should be implemented as part of that 3rd integration. Then an additional PR or two will also need to be developed to migrate existing integrations to the core/common framework.