Make provider class mandatory in DB

[dmjb]

Relates to #2845

Undo migration #35 after discussion with Ozz and Jakub this morning. We want the provider class to store the concrete type of the provider, this will be used by the ProviderFactory.

After studying the code, there does not appear to be any situation in which this field will be null in the staging/prod DB.

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[coveralls]

coverage: 48.162% (+0.02%) from 48.143%
when pulling 7902979 on provider-class-mandatory
into 9139c73 on main.