[keyserver] Create a table for invite links

Summary: This new table will be used to store links and collect data about their usages.

Test Plan: Run migration and check if the table was created. Create a new db and check if that also creates this table.

Reviewers: bartek, kamil, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7519