[web] introduce panel header component

Summary:
As I have been working on the redesign and have been converting our ui to utilize the new Panel component, I noticed that a lot of the headers followed a similar ui pattern. Rather than have this pattern be copied and pasted a bunch, I created this PanelHeader component that can be used to factor and abstract that logic out. The pattern I was noticing with the panel headers was that many panel headers either would just have the label, or they would have an optional add button.

Test Plan: flow + confirmed that there were no regressions with the app list header

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10610