Implement the majority of the Users page, shown when clicking "manage other users...

Implement the majority of the Users page, shown when clicking "manage other users" in Chrome OS' People settings.

Still TODO:
1. Do client side / JS validation of the user name / email entered in the "Add users" field.
2. Add the desired prefs to the whitelist in prefs_util.cc.
3. Use an inverted pref (we need to make this first in a separate cl) for the 'restrict sign-in to the following users' checkbox.
4. Gray out checkboxes / fields when user isn't the owner, and hide the user list when the settings isn't managed.
5. Prevent the owner from deleting herself/himself.
6. Verify that everything works right with the fully implemented usersPrivate API.
7. Make the Users page a subpage rather than a top level page. I'm making it a top level page for now since otherwise there's no way to access it. Once we add the top level People page we'll move this underneath it.

BUG=495858

Review URL: https://codereview.chromium.org/1145423008

Cr-Commit-Position: refs/heads/master@{#332876}