GitHub scope upgrade UX issues

Created by: ryanslade

A few small UX issues left over from https://github.com/sourcegraph/sourcegraph/pull/21495

• Make the update button purple as in the designs
• Display notice without requiring a page refresh
• When button text is replaced by spinner, the button shrinks. Can we keep it the same size?

Part of: https://github.com/sourcegraph/sourcegraph/issues/20229