[Cashtab] Export contact list

Summary:
Supersedes D11321.

Contact export function.

Test Plan:
open a wallet with an empty contact list and ensure the Export contacts button is not rendered
add a new contact, verify the Export contacts button is displayed and ensure clicking on this button results in a csv file being generated for download
add two more contacts and ensure the exported csv file shows each contact delimitered by a comma
test across browsers and extension/iOS/Android

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11342