Allowlist for Application Link disable incoming traffic by default causing Confluence fail to parse External Gadget on each Restart

Issue Summary

Allowlist for Application Link disable incoming traffic by default causing Confluence fail to parse External Gadget on each Restart

This is reproducible on Data Center: yes

Environment

Tested and confirmed the issue still exist in Confluence 8.5.4 AppLinked with Jira 9.12.1

Steps to Reproduce

1. Install Confluence and Jira (latest LTS was used in this test)
   • Confluence 7.13.5
   • Jira 8.20.7
2. Establish Applicaiton Links between Confluence and Jira
3. Register External Gadget in Confluence using Jira URL
   
4. Restart Confluence and pay attention to the startup log

Expected Results

Confluence is able to request for the external gadget inforamtion and parsed it correctly

Actual Results

Error messages where Confluence failed to parse external gadgets like below can be found in the startup log:

2022-04-03 11:35:33,545 WARN [AtlassianEvent::CustomizableThreadFactory-5] [renderer.internal.http.HttpClientFetcher] fetch A request to http://localhost:48203/j8203/rest/gadgets/1.0/g/com.pyxis.greenhopper.jira:greenhopper-gadget-version-report/gadgets/greenhopper-version-report.xml has been denied. To allow requests to this URL add the application URL to your whitelist (http://confluence.atlassian.com/x/KQfCDQ).
...
2022-04-03 11:35:33,546 WARN [AtlassianEvent::CustomizableThreadFactory-5] [plugins.gadgets.metadata.GadgetsMacroMetadataProvider] generateMacroMetadata Unable to parse gadget spec [ http://localhost:48203/j8203/rest/gadgets/1.0/g/com.pyxis.greenhopper.jira:greenhopper-gadget-version-report/gadgets/greenhopper-version-report.xml ]: org.apache.shindig.gadgets.GadgetException: Unable to retrieve gadget xml. HTTP error 403

This also cause an issue with the External Gadgets not showing up in Macro Browser.

Workaround

Modify the Allowlist for the Application Link to include incoming traffic:

Since Confluence will always load the external gadgets from Jira on startup, restarting Confluence after making the change above will load the gadgets in