Add __str__ method to Banner model

[Scotchester]

This will display the title of the banner in various places in the Wagtail admin, instead of generic text like "Banner object (2)".

Additions

• __str__ method added to Banner model

Testing

With a recent database dump:

1. Visit http://localhost:8000/admin/v1/banner/edit/2/ and see that the header reads: "EDITING Banner object (2)"
2. Pull branch
3. Refresh the page and see that the header now reads: "EDITING Interagency mortgage and housing assistance pages"

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the CFPB development guidelines
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated
• Reviewers requested with the Reviewers tool ➡️