Revert "Remove `Module_Sharing_Settings->register()` from `Modules` class."
Revert "Remove `Module_Sharing_Settings->register()` from `Modules` class."

This reverts commit 0d833c6e3e17d54303ae450f216647d8bf35b051.

google/site-kit-wp,google/site-kit-wp,google/site-kit-wp,google/site-kit-wp