[keyserver] Allow routing to invite endpoint

Summary: When landing page is configured with / base path, it receives all the requests before /invite/:secret can be handled - this route should be declared before any other router.

Test Plan: Configure landing page so that it is server on /. Open /invite/abc route and check if a user is redirected to {commBasePath}/handle/invite/abc. This testing procedure still is far from our prod setup, but should be enough to check if the solution is correct.

Reviewers: bartek, kamil, inka

Reviewed By: bartek, kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8209