feat(unlock-app, locksmith): better referrers set!

[julien51]

Description

When creating or saving a checkout config, we use its creator by default as the referrer if none is set.
This means that the wallet setting up an event will receive UDT by default, and anyone creating a checkout config (URL) will also be the default referrer.

Fixes #14045

Checklist:

• 1 PR, 1 purpose: my Pull Request applies to a single purpose
• I have commented my code, particularly in hard-to-understand areas
• I have updated the docs to reflect my changes if applicable
• I have added tests (and stories for frontend components) that prove my fix is effective or that my feature works
• I have performed a self-review of my own code
• If my code involves visual changes, I am adding applicable screenshots to this thread

Release Note Draft Snippet