[keyserver] modify thread creator + deleter endpoints to update communities table

Summary:
With our new communities table on the keyserver we want to make sure that if a root level thread is created or deleted (a communitiy) that we also create + delete the respective row in the communities table.

Depends on D11903

Test Plan: Created a thread through the community creation user flow + confirmed that a new row was inserted in both the threads and communities tables. Deleted that root thread (communitiy) and confirmed that the correct row was removed in both the threads table and the communities table

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11904