Enable integration of doccano within different web services.

Feature description
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Hi there, I'm extremely excited to see an open-source text annotation tool here. Thanks!

One of my projects needs to provide active learning as a service to users. I wish I could adopt the workflow of doccano in our service instead of redirecting users to a doccano endpoint. For example, if there is an annotation task, I would like to show the doccano's annotation UI on our website. (SSO integration would be sweat too.)

I understand it might not be in the current roadmap, but if this feature is within the scope, I'd be happy to put engineering effort here.

Thanks!