Update plugin assets

[jeffpaul]

Description of the Change

This PR adds in missing banner images (to display when someone clicks the View details link from the WP Admin plugin screen) and updates the icon images to match. Note this is currently WIP while I await an updated icon.svg file, I'll update the PR when that's available.

Alternate Designs

n/a

Benefits

Ensures we properly display ClassifAI imagery where possible.

Possible Drawbacks

none identified

Verification Process

1. Open wp-admin/plugins.php
2. Click "View details" next to installed ClassifAI plugin
3. View updated images

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

n/a

Changelog Entry

### Changed
- Updated plugin icon, added banner (props [@blancahong](https://profiles.wordpress.org/blancahong/)).