Spec coverage and refactor for the api/v1/accounts controllers (#3451)

app/controllers/api/v1/accounts/credentials_controller.rb

...ntrollers/api/v1/accounts/follower_accounts_controller.rb

...trollers/api/v1/accounts/following_accounts_controller.rb

app/controllers/api/v1/accounts/relationships_controller.rb

app/controllers/api/v1/accounts/search_controller.rb

app/controllers/api/v1/accounts/statuses_controller.rb

app/controllers/api/v1/accounts_controller.rb

app/views/api/v1/accounts/relationships/index.rabl

app/views/api/v1/accounts/statuses/index.rabl

config/routes.rb

...ontrollers/api/v1/accounts/credentials_controller_spec.rb

...lers/api/v1/accounts/follower_accounts_controller_spec.rb

...ers/api/v1/accounts/following_accounts_controller_spec.rb

...trollers/api/v1/accounts/relationships_controller_spec.rb

spec/controllers/api/v1/accounts/search_controller_spec.rb

spec/controllers/api/v1/accounts/statuses_controller_spec.rb

spec/controllers/api/v1/accounts_controller_spec.rb

spec/routing/api_routing_spec.rb