Update plugin's imagery and branding

[CGastrell]

Current .org plugin page holds outdated images and icons since the rename/rebranding:

Plugin page: https://wordpress.org/plugins/polldaddy Trello card: https://trello.com/c/2YFM07xU/15-update-design-for-plugin-store-page Ref: https://developer.wordpress.org/plugins/wordpress-org/plugin-assets/

@digitalwaveride Mentioning to keep track of this

Preview can be attained by appending ?banner_url=A_URL_TO_YOUR_IMAGE

[roundhill]

@CGastrell we can close this one, I think?

[CGastrell]

Indeed, thanks!