Add OAuth2 authentication (#147)

- Added Site settings > Authentication section
- Create/edit/delete your custom oauth2 configurations
- Login or signup with oauth2

app/workflows/OAuthExchangeAuthCodeForProfile.rb

@@ -0,0 +1,56 @@
+class OAuthExchangeAuthCodeForProfile
+  include HTTParty
+
+  attr_accessor :authorization_code, :o_auth
+
+  # Given:
+  #   authorization_code: code returned by OAuth provider on user confirmation
+  #   o_auth: ActiveRecord model with information about the OAuth provider
+  #
+  # The workfow first exchanges the authorization code for an access token
+  # Then it uses the access token to fetch user profile information
+  #
+  # Returns:
+  #   The fetched user profile as a Hash, if successful
+  #   nil, if unsuccessful
+
+  def initialize(authorization_code: "", o_auth: "")
+    @authorization_code = authorization_code
+    @o_auth = o_auth
+  end
+
+  def run
+    return nil unless @o_auth and @o_auth.class == OAuth
+    return nil unless @authorization_code and @authorization_code.class == String
+
+    begin
+      # Exchange authorization code for access token
+      token_request_params = {
+        code: @authorization_code,
+        client_id: @o_auth.client_id,
+        client_secret: @o_auth.client_secret,
+        grant_type: 'authorization_code',
+        redirect_uri: @o_auth.callback_url
+      }
+      
+      token_response = HTTParty.post(
+        @o_auth.token_url,
+        headers: { "Accept": "application/json" },
+        body: token_request_params
+      )
+      access_token = token_response['access_token']
+      
+      # Exchange access token for profile info
+      profile_response = HTTParty.get(
+        @o_auth.profile_url,
+        headers: { "Authorization": "Bearer #{access_token}" },
+        format: :json
+      ).parsed_response
+
+      return profile_response
+    rescue => error
+      print(error)
+      return nil
+    end
+  end
+end