feat: update some fields in customer agreemnet model

[jajjibhai008]

Description

I have made the following updates in the UserAgreement Model

• Add a net-new field called, Modal Header Text
• Changed Expired Subscription Modal Messaging to an HTML field, rather than text (Why: So we can emphasize bolding or different color text in the body of the modal)
  • Added sanitization on the field to make it secure by using a bleach package
• Added a new field Button Label in Modal
• Added a new field URL for Button in Modal
• Update the serializer to expose the newly added fields

JIRA Ticket - > ENT-9615

Testing considerations

• Include instructions for any required manual tests, and any manual testing that has
  already been performed.
• Include unit and a11y tests as appropriate
• Consider performance issues.
• Check that Database migrations are backwards-compatible

Post-review

Squash commits into discrete sets of changes