[Cashtab] Toggle "buy nfts" and "manage nfts" for better user presentation

Summary:
Load the NFT screen showing NFTs that are available to buy by default, taking up all available screen area.

Include a toggle switch to NFTs listed by the active wallet.

This corrects annoying "double scroll" behavior from trying to fit too much information on a single screen by the use of panels.

It is still useful to load all the information in the same screen, since we need all of this agora information to separate the data.

Test Plan:
npm test

check out this diff on test site at https://cashtab-local-dev.netlify.app/#/nfts

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: emack, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D16759