Change chat tab names to 'Focus' and 'Background'

Summary: Changed the names of the chat tabs on the web app from 'HOME' and 'BACKGROUND' to 'Focus' and 'Background'. First diff, hope I did this right.

Test Plan: Run web app to make sure names are changed properly.

Reviewers: palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D2219