[keyserver] Remove target blank

Summary: Unfortunately, it doesn't solve https://linear.app/comm/issue/ENG-4089/invite-links-dont-work-on-prod.

Test Plan: Check if clicking a link in Chrome doesn't open a new card.

Reviewers: kamil, inka

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8486