[e.cash] Rename "ecash-brand" page to "brand"

Summary:
e.cash/brand is a better url, e.cash/ecash-brand is redundant.

Also redirect e.cash/ecash-brand in case there are external links out in the wild.

Test Plan:
npm run dev
Check that navigation links go to localhost:3000/brand
Type in localhost:3000/ecash-brand and check that it redirects to localhost:3000/brand`

Reviewers: johnkuney, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14367