Allow learners to sign up for new accounts on Learner only devices when the facility allows it

[rtibbles]

Because of the extra technical complexity of allowing learners to sign up on a learner only device, it is currently disabled.

To allow the most flexibility for learner sign up, we should let learners sign up on learner only devices, then let that learner be signed up and properly attached to the facility when an appropriate full facility device is discovered.

This would probably need to use similar mechanisms to the merge_users function: https://github.com/learningequality/kolibri/blob/release-v0.15.x/kolibri/core/auth/utils.py#L50 as the created user would need to be recreated in the full facility device, synced to the learner only, and then merged with the previously created learner.

If we're disabling this to save time, maybe we should open a follow-up issue to re-enable it later?

This is blocked because it's a technical limitation - the user account has to originate from the full facility.

In order to allow this to work we'd have to have a more complex technical workflow behind the scenes that somewhat replicates what we have done for the 'account merging' for migrating 0.14 devices to learner only devices.

Happy for us to open a follow up issue, but as I noted in the issue being fixed here, this is the practicable short term fix for 0.15.0.

Originally posted by @rtibbles in https://github.com/learningequality/kolibri/issues/8385#issuecomment-912904442