[Cashtab] Button labeling & positioning in Contact List patch

Summary: Related to T2411. Add Contact and Export Contacts buttons have been modified, icons added and positioning has been changed.

Test Plan:
cd web/cashtab && npm start
navigate to the settings tab
expand the Contact List collapse
observe that:
Export Contacts and New Contact buttons have been changed to readDownload CSV & Add Contact
Icons have been added
Buttons are positioned next to each other

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11528