FEATURE: Make auth_redirect param options on user_api_keys

This is a possible solution for https://meta.discourse.org/t/user-api-keys-specification/48536/19
This allows for user-api-key requests to not require a redirect url.
Instead, the encypted payload will just be displayed after creation  ( which can be copied
pasted into an env for a CLI, for example  )

Also: Show instructions when creating user-api-key w/out redirect

This adds a view to show instructions when requesting a user-api-key
without a redirect. It adds a erb template and json format.
Also adds a i18n user_api_key.instructions for server.en.yml

spec/requests/user_api_keys_controller_spec.rb

@@ -205,5 +205,40 @@ describe UserApiKeysController do
 
       expect(response.status).to eq(302)
     end
+
+    it "will just show the payload if no redirect" do
+      user = Fabricate(:user, trust_level: 0)
+      sign_in(user)
+
+      args.delete(:auth_redirect)
+
+      SiteSetting.min_trust_level_for_user_api_key = 0
+      post "/user-api-key", params: args
+      expect(response.status).not_to eq(302)
+      payload = Nokogiri::HTML(response.body).at('code').content
+      encrypted = Base64.decode64(payload)
+      key = OpenSSL::PKey::RSA.new(private_key)
+      parsed = JSON.parse(key.private_decrypt(encrypted))
+      api_key = UserApiKey.find_by(key: parsed["key"])
+      expect(api_key.user_id).to eq(user.id)
+    end
+
+    it "will just show the JSON payload if no redirect" do
+      user = Fabricate(:user, trust_level: 0)
+      sign_in(user)
+
+      args.delete(:auth_redirect)
+
+      SiteSetting.min_trust_level_for_user_api_key = 0
+      post "/user-api-key.json", params: args
+      expect(response.status).not_to eq(302)
+      payload = JSON.parse(response.body)["payload"]
+      encrypted = Base64.decode64(payload)
+      key = OpenSSL::PKey::RSA.new(private_key)
+      parsed = JSON.parse(key.private_decrypt(encrypted))
+      api_key = UserApiKey.find_by(key: parsed["key"])
+      expect(api_key.user_id).to eq(user.id)
+
+    end
   end
 end