Add accredited column to Provider

[inulty-dfe]

Context

This column will replace the accrediting_provider column

The purpose of the accrediting_provider is to denote whether the provider has been given accredited status by DfE.
This column causes frustration for developers.

• The column is on the provider table and it has a suffix _provider
• It stores a text value which is one of two values and those values are a single byte each
• We are confused by accredited_provider, Course#accrediting_provider, Provider#accrediting_provider
• We don't know whether to use ``Provider.accrediting_provider == 'accredited_provider'orProvider#accredited_provider?`
• There are tests like this expect(provider.reload.accredited_provider).to eq('not_an_accredited_provider')

Changes proposed in this pull request

Add a new column to replace the accrediting_provider column

Guidance to review