[remove_access_token] Only store the access_token in memory.

Summary:
[remove_access_token] Remove obsolete columns from model.

[remove_access_token] Update code to only store access tokens in memory.

[remove_access_token] Migration script.

Fixes T103

Test Plan: Review code, run regression

Reviewers: emfree

Reviewed By: emfree

Subscribers: spang, mg

Maniphest Tasks: T103

Differential Revision: https://review.inboxapp.com/D217