[web] Combine constants in web/assets.js into objects

Summary:
Addresses feedback from @palys-swm in D4041.
Linear Issue: https://linear.app/comm/issue/ENG-1172/combine-constants-in-webassetsjs-into-objects

Test Plan:

1. Loaded the web app
2. Navigated to background tab and ensured that illustration looked as expected
3. Navigated to NotificationsModal and ensured that illustrations look as expected

Reviewers: palys-swm, def-au1t, benschac

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh, palys-swm

Differential Revision: https://phabricator.ashoat.com/D4107