[keyserver] Create an endpoint that allows disabling links

Summary:
At this point calling the endpoint results in link being deleted. In the future we can consider having a list of disabled links.

Depends on D7885

Test Plan: Tested in combination with the rest of the stack. Confirming disabling a link results in link being deleted.

Reviewers: bartek, kamil, inka

Reviewed By: kamil

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D7906