Use state if existing in session

[coel]

Explain your changes

When generating multiple links, the state of earlier links are overridden so do not successfully complete login on callback. Specifically, where login is started in multiple tabs, only the last one can be used successfully to login. These changes reuse state until successful login, so any of the tabs can be used to login.

Note: After login, the remaining tabs will still not be able to be used for login. The state can still be passed in if a new state is preferred on every call.

Alternative solution to: #40

Checklist

• I have read the “Pull requests” section in the contributing guidelines.
• I agree to the terms within the code of conduct.

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