Unlinking stored credential - soft delete

[Dmitri Zagidulin]

When user presses 'unlink' button on LCW:

1. Add an API endpoint DELETE /api/credentials/${credentialId}
2. Make sure it's a "soft delete" (flips a flag in the db, not an actual remove from db).

[Dmitri Zagidulin]

Implemented in PR #33, closing.