[#923] OAuth consumer mode refactoring, new tests, tests adjustments, readme.

config/config.exs

docs/config.md

lib/pleroma/config.ex

lib/pleroma/web/endpoint.ex

lib/pleroma/web/oauth/fallback_controller.ex

lib/pleroma/web/oauth/oauth_controller.ex

lib/pleroma/web/templates/o_auth/o_auth/consumer.html.eex

lib/pleroma/web/templates/o_auth/o_auth/show.html.eex

test/registration_test.exs

test/web/oauth/oauth_controller_test.exs