[e.cash] Properly use constants for the URLs

Summary:
Consistently use the constants:

• NEXT_PUBLIC_SITE_URL for the main URL
• NEXT_PUBLIC_STRAPI_URL for the strapi URL
• NEXT_PUBLIC_STRAPI_SCORECARD_URL for the scorecard URL

Ref T3904.

Test Plan:

npm run dev

Check the site works as normal

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T3904

Differential Revision: https://reviews.bitcoinabc.org/D18798