[web] introduce add button component

Summary: The new web app design uses this add button in several places. Rather than copy + paste this logic in all those places, I built this component that can be reused where we need this add button.

Test Plan: flow + confirmed that the prexisting add button in the keyserver selection list has no regressions

Reviewers: atul, rohan, kamil

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10436