feat(payments-server): Show coupon name in in Subscription Management

[xlisachan]

This pull request

• Shows the promotion information applied to the subscription, if present, from the subscription and billing endpoint.

Note: No localization is currently needed, but it has been included in this PR anyways.

Issue that this pull request solves

Closes: FXA-9457

Checklist

Put an x in the boxes that apply

• [x] My commit is GPG signed.
• [x] If applicable, I have modified or added tests which pass locally.

Screenshots

Without coupon information

Before

Before (with display name)

Mock-up

After

With coupon information

Mock-up

After

After (with display name)