[Clank CBCM] Make learn more link clickable on chrome://management

This CL introduces a ManagementMediator class, which is responsible for
handling business logic in the Clank MVC model and communicate with
libraries external to the component (in this case, the one used for
opening the HC article page).

It also fixes the URL to use a p-link, as recommended by a presubmit
check.

Screen recording: https://bugs.chromium.org/p/chromium/issues/detail?id=1277538#c5

Bug: 1277538
Change-Id: Idd4091192ca40c46f7ad19c268edb86d2ab81370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3343915
Commit-Queue: Fabio Tirelo <ftirelo@chromium.org>
Auto-Submit: Fabio Tirelo <ftirelo@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#953068}