Add onCopy callback to CopyableText

Created by: philipp-spiess

Part of https://github.com/sourcegraph/sourcegraph/issues/27101

This PR is based on top of https://github.com/sourcegraph/sourcegraph/pull/30591 to avoid merge conflicts (we'll land them together anyway).

In order to log events around when invite links are copied, we need a callback from the <CopyableText /> component. Notice that there are two paths to copying text: Either by clicking the button or by focusing the text. This onCopy callback is emitted on both paths.

I tested the changes by adding an onClick={() => console.log("onCopy called")} prop to the caller in StartSearching.tsx and it worked as expected: