Change API documentation method to drop Flask-API

We currently use Flask-API to have browsable API endpoints which provide documentation if a user submits a correctly formatted request in-browser, and used to provide documentation for base API endpoints (/api/1/search/, /api/1/directory/ ...) until we removed default parameters in the api view.

We would like to find a more customizable way of providing browsable endpoints in general, to replace Flask-API, which leaks into the browsing view and isn't sufficiently adaptable to allow us to document base endpoints in a consistent manner. The 'api.html' template would be a good starting point to generating the documentation on our own, should we choose to do so..

Migrated from T431 (view on Phabricator)

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