Replace setting custom CORS headers with rack-cors, set it on /oauth/token endpoint

app/controllers/api_controller.rb

@@ -7,7 +7,6 @@ class ApiController < ApplicationController
   skip_before_action :verify_authenticity_token
 
   before_action :set_rate_limit_headers
-  before_action :set_cors_headers
 
   rescue_from ActiveRecord::RecordInvalid do |e|
     render json: { error: e.to_s }, status: 422
@@ -50,13 +49,6 @@ class ApiController < ApplicationController
     response.headers['X-RateLimit-Reset']     = (now + (match_data[:period] - now.to_i % match_data[:period])).to_s
   end
 
-  def set_cors_headers
-    response.headers['Access-Control-Allow-Origin']   = '*'
-    response.headers['Access-Control-Allow-Methods']  = 'POST, PUT, DELETE, GET, OPTIONS'
-    response.headers['Access-Control-Request-Method'] = '*'
-    response.headers['Access-Control-Allow-Headers']  = 'Origin, X-Requested-With, Content-Type, Accept, Authorization'
-  end
-
   def set_pagination_headers(next_path = nil, prev_path = nil)
     links = []
     links << [next_path, [['rel', 'next']]] if next_path