SHS-5941: Install CKEditor anchor_link plugin

[cienvaras]

READY FOR REVIEW

Summary

• Install and configure anchor_link module

Need Review By (Date)

...

Urgency

medium

Steps to Test

1. Create a Flexible Page
2. Confirm that CKEditor includes a new button in the toolbar for adding anchors (see image below)
3. Test the anchor functionality
   • Select some test, add an anchor with some name (for example anchor-test)
   • Add some more text and then a link, pointing to the anchor (#anchor-test)
   • Save the page and confirm that the anchor link works as expected. You can also inspect the page to confirm that the anchor markup is being added
4. Edit the page again and confirm that the rest of the functionality in CKEditor is unchanged and works as expected.

PR Checklist

• PR Checklist
• Humsci Basic PR Checklist