[web] [refactor] use assetUrl variable

Summary:
swap hard coded url for assetUrl variable like the rest of the project uses.

Test Plan: go to the keyserver page. arrow should render.

Reviewers: atul, palys-swm, def-au1t, yayabosh

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3611