Add code host icons to 'Open in [code host] button

Created by: AlicjaSuska

Part of RFC 221: Improve discoverability/awareness of the browser extension

Description

• Currently, we display the code host logo on for 'Open on GitHub' and a generic icon for the remaining code hosts

• Requested change: add code host logos to improve discoverability

Design documentation

Logos of GitLab and Bitbucket Server are available in the standard icon set. Phabricator needs a custom icon - available in Figma for download.

Stats

• Track the clicks on those buttons @ebrodymoore FYI