DCJ-654: Standardize metrics token usage

[rushtong]

Addresses

Partially addresses https://broadworkbench.atlassian.net/browse/DCJ-654

Summary

This PR pulls out a small part of #2611 to consolidate how we're getting the access token for Metrics (bard) ajax requests. This will simplify the full B2C sign-in PR here: #2667

Drive-by fixes:

• Typo in acknowledgements.js
• Unused getFeatureFlag function
• Unused fileOpts function
• Unused fileBody function
• Reformatted with eslint

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[fboulnois]

lgtm 👍