VACMS-10356 Add content validator for Preview URL links (#10356)

[JunTaoLuo]

Description

Closes #10356.

Testing done

• Added unit tests for validator
• Tested functionality locally

Screenshots

QA steps

What needs to be checked to prove this works?
What needs to be checked to prove it didn't break any related things?
What variations of circumstances (users, actions, values) need to be checked?

As user with ability to edit content:

1. Edit any node/page with rich text field, e.g. the VAMC system service description of a System Health Service.

2. Insert a link to a preview URL, e.g. <a href="https://preview-staging.vfs.va.gov/example/path">Example Link!</a>

3. Try to save the document and be blocked by errors and validate that:

   • An error appears at the top of the page:
   
   • The field title is highlighted red to indicate error:
   
   • An error appears at the bottom of the field:
   

4. Edit any node/item with plain text field, e.g. description of a media image.

5. Add a link to a preview URL, e.g. https://preview-staging.vfs.va.gov/example/path

6. Try to save the document and be blocked by errors and validate that:

   • An error appears at the top of the page:
   
   • The field title is highlighted red to indicate error:
   
   • An error appears at the bottom of the field:
   

7. Then validate Acceptance Criteria from issue

   • A link created to the preview server url will fail form validation and prevent save.
   • Validator is covered by unit tests.

Definition of Done

• Documentation has been updated, if applicable.
• Tests have been added if necessary.
• Automated tests have passed.
• Code Quality Tests have passed.
• Acceptance Criteria in related issue are met.
• Manual Code Review Approved.
• If there are field changes, front end output has been thoroughly checked.

Select Team for PR review

• CMS Team
• Public websites
• Facilities
• User support
• Accelerated Publishing

[ndouglas]

MY MY MY, THIS PULL REQUEST IS JUST AS PRETTY AS A PICTURE!

Great job!

[JunTaoLuo]

@ndouglas Oops, I didn't realize pushing a new commit to fix a comment dismisses your review.

[ndouglas]

@JunTaoLuo NP, some repos here have that branch protection rule set (like this one), some don't (devops and content-build, notably).