Invenio-oauth2server

- Removes updating the ``expires`` for personal tokens.
- Removes ``OAUTH2_PROVIDER_TOKEN_EXPIRES_IN`` from configuration.

- Restrict oauthlib to latest v2.
- Restrict requests-oauthlib lower than 1.2.0 because of oauthlib 3.

- Fix incosistent OAuth2 state initialization between UI and REST applications.
- Basic token management CLI commands for creating/deleting personal access
tokens.
- Better token creation warning messages.

- Flask v1.0 support.

- Initial public release.