[web] Added more utility functions for typeahead.

Summary:
Added utility functions for calculating overlay position and
another one for generating actions that will end up in overlay.

Test Plan:
Functions visible, but not called anywhere yet.
Will test in final diff.

Reviewers: tomek, atul, kamil

Reviewed By: tomek

Subscribers: ashoat, kamil, atul, tomek

Differential Revision: https://phab.comm.dev/D5719