Problem/Feature

Adding tiny open envelope icon for threads open tracking:

Guidelines

Make sure the pull request:

[ ] Follows the established folder/file structure
[ ] Adds unit tests
[ ] If it is a refactor or change to an existing component, have you verified it won't break existing Cypress tests or have you updated them?
[ ] Did you verify some accessibility (a11y) basics?
[ ] Adds/updates stories. Guidelines
[ ] Adds/updates documentation (ie proptypes) Guidelines
[ ] Has it been tested in Help Scout's supported browsers?
[ ] Requests review from designer of the feature
[ ] Add label (bug? feature?)