Define type for <F>(serverCall: ActionFunc<F>) => F

Summary: Linear issue: ENG-1845. Defining a type for <F>(serverCall: ActionFunc<F>) => F for code clarity.

Test Plan: See that it is still possible to remove a user as a friend.

Reviewers: tomek, michal, jakub, marcin

Reviewed By: tomek

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5215