[keyserver] Handle web invite links on keyserver

Summary:
This is a really simple diff that achieves a lot.
After clicking an invite link on web a user is redirected to the app. In the app, this new route isn't implemented, so the user either sees a login screen or a place which was opened in the most recent session, which is exactly what we want.

Instead of simply using ${baseRoutePath}invite/${secret} I had to create something different, because in case of an empty base path, we would get a redirect loop.

Test Plan:
Open localhost:3000/invite/abc on web - it should be redirected to localhost:3000/comm/handle/invite/abc and then e.g. to localhost:3000/comm/.
On iOS accepting invite flow should still work.

Reviewers: bartek, kamil, inka

Reviewed By: kamil

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7948