Move generic django authentication code to swh-auth

swh-web implements several django backends and views to authenticate a user using OpenID Connect and Keycloak.

Those backends and views are generic and could be used in other django applications.

Recently, the rDAUTH repository has been created to share common authentication code between swh-deposit and swh-web in a new swh-auth module.

So generic django authentication code in swh-web should be moved to the swh-auth module in order to ease reusability and remove code duplication between the two modules.

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Migrated from T3150 (view on Phabricator)