[keyserver] Serve .well-known from landing endpoint
Summary:
In mobile app we're using comm.app as deep links. To do that, we need to serve .well-known on this url (which is associated with our landing page).
Test Plan: Check if /landing/.well-known/assetlinks.json returns a proper file.
Reviewers: kamil, inka, bartek
Reviewed By: bartek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D8357