[Cashtab] Improve form entry UX for agora listings

Summary:
It is annoying and confusing to have '0' in this field by default, since when you start typing, you begin with more than you expect. e.g. you type "1" but you get "10" because of the existing zero. Or if you clicked after the zero, you get "01" (weird).

Make all form entries strings. Initialize as empty.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17649