Periodic points hook: save the period usermeta separate in multisite when not network active

Otherwise, the periods for different sites would get tangled.
Uninstall: delete periodic points hooks metadata

Also related changes for uninstall unit tests.

See 30645b21d21b7d13078135b4e7050c30e879c9fa
Uninstall unit tests: add points to a user in the usage simulation
Squashed 'tests/phpunit/includes/uninstall/' changes from a9a0ce2..6845eb6

6845eb6 Add no-user-options-with-prefix assertion

git-subtree-dir: tests/phpunit/includes/uninstall
git-subtree-split: 6845eb6ed165f66b30c46304119a1eb0ab9f17c4
Uninstall unit tests: update as required for a4b2403d44ce21e917c30946b4c4c81567f1ed83

Merges commit '994bd0d80a192d28beabafdf14e6c713a6ddf1e8'