Center loading icon

Summary: Centering the antd loading icon. Added two overridding css items in the app.js component in order to adjust the antd styling

Test Plan: run the app and see if the loading icon looks more centered. Could force it to always be on to see more closely

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D11882