[QA-1326] Update challenge button labels to match spec

[dharit-tan]

Description

Designs have changed a bit since the last time we touched this code - this PR brings the button labels up to spec by adding a completedLabel field to the challenge config.

How Has This Been Tested?

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 4dcc51448c385586a3bcfa0a239e62c4a54426d7

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[audius-infra]

Preview this change https://demo.audius.co/rt-challenge-completed-label