[e.cash] Retrieve the latest version from github and use it on the upgrade page

Summary: Fetch the latest version from the Github API.

Test Plan:

npm install
npm run dev

Check the latest version is displayed.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15908