[keyserver] Fix landing SSR

Summary:
Context here: https://linear.app/comm/issue/ENG-2511#comment-ec66b35a

Stripping the slash doesn't matter on web, where we don't actually use react-router for routing (we define just one route and parse the URL ourselves). However, on landing we use react-router directly, so we should make sure we don't have an extra slash.

Test Plan: Load http://localhost:3000/commlanding/keyservers and check the console log to make sure there are no SSR errors

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D5970