Introduce referral program

[justinrobots]

Description

For users on a paid plan:

• Display a dismissible info card at the top of Home that tells users about the referral program. It should include a message and a CTA that takes users to the referral program page on gk.dev
• Add a GitLens: Refer a friend command that opens the referral program page on gk.dev
• Add a GitLens: Copy my referral link command that copies the referral link for this user into the clipboard so they can paste it somewhere else to share
• In the Home ellipsis menu, add an option to "Refer a friend" that opens the referral program page on gk.dev.