fix(views): input/userpicker can now remove all users on edit

We must provide an empty input in case all users are removed. Otherwise
no value will be submitted.

Fixes #6982
fix(plugins): trigger plugin hooks when saving plugin settings

The introduction of setPrivateSetting led to failing to trigger the plugin hooks.
Only property sets result in the hooks being triggered so we return to that method.

Fixes #6820
Merge pull request #7471 from mrclay/lowfill-1.9

fix(plugins): trigger plugin hooks when saving plugin settings
Merge pull request #7470 from mrclay/6982_19_userpicker

fix(views): input/userpicker can now remove all users on edit