[keyserver] Add ignorePermissions option to deleteThread

Summary:
This is needed in the following diff so that we can call deleteThread from a migrations context, where we use a viewer that otherwise doesn't have permissions to delete any thread (createScriptViewer(bots.commbot.userID)).

I modeled it after the equivalent permission in updateThread.

Test Plan: Before this diff, the following diff errored with an invalid_credentials error. After this diff, it no longer does.

Reviewers: atul, rohan, inka

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9229