[keyserver] Add background and logo images

Summary:
Display background image - cover the container if aspect ratio of device is different from image's. Also Comm logo is displayed.

Depends on D7330

Test Plan: Open the page on simulator and in desktop browser and check if images are displayed.

Reviewers: bartek, kamil

Reviewed By: kamil

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7348