Email like usernames disallowed in all circumstances

[rtibbles]

Observed behavior

Currently email like usernames are disallowed via the Validation logic set in the backend: https://github.com/learningequality/kolibri/blob/release-v0.16.x/kolibri/core/auth/models.py#L367 this precludes the generation of FacilityUsers with email like usernames, unless created directly through the ORM (without doing a full_clean of the model prior to save), and also prevents their syncing, as Morango does a full_clean of the model during the deserialization process.

Expected behavior

There are certain circumstances where this is desirable - the main one being for use with the oidc-client-plugin, where the system being used for Open ID verification is using emails as the usernames.

To achieve this, but not cause a drastic change and a slew of non-backwards syncable FacilityUser accounts, we should move the username validation regex out of the model definition, and into the FacilityUserSerializer: https://github.com/learningequality/kolibri/blob/release-v0.16.x/kolibri/core/auth/serializers.py#L79

This will allow the OIDC client plugin to create these usernames, as it uses the create_user method of the FacilityUser manager class: https://github.com/learningequality/kolibri-oidc-client-plugin/blob/master/kolibri_oidc_client_plugin/auth.py#L83

The one side effect here is that users of this type will also be able to be created via the command line using management commands.

User-facing consequences

Difficulty integrating with systems where email is the primary username.