Introduce referral program

[justinrobots]

For users on a paid plans only:

• Add a GitLens: Refer a friend command that opens the referral program page on gk.dev
• In the Home account menu, add an option to "Refer a friend" that opens the referral program page on gk.dev.

Referral page on GKDev: https://gitkraken.dev/?referral_portal=true&source=gitlens

[github-actions]

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