Im/add routes and controllers for provider partnerships

[inulty-dfe]

Context

Add the routes, controllers and views required for enabling provider partnerships.

Provider partnerships is the rebuilding of the associations between Lead partners and Accredited providers.

This PR is for testing and feedback. It will likely be merged in incremental PRs

Changes proposed in this pull request

This PR tries to create all the code necessary for the old implementation to be swtiched seamlessly to the new.

1. Use routing constraint to enable/disable routes
2. The constraint currently depends on a Settings
3. Create a v2/Training providers becuase I don't see the name of this model / routing changing? Maybe Training partners in fact
4. Create a ProviderPartnershipsController
5. This changes the URL path
6. The Accredited providers implementation depends on a search box "accredited_provider_search" - it doesn't seem logical to rename this to "provider_partnership_search"
7. This controller is awkward and requires conditionals to exist while both implementations exist.

Guidance to review