[JENKINS-18884] Added configuration for access to "People" page.

People page, which displays all users registered to Jenkins can be accessed any user with Overall/READ permission.
Especially when the Overall/READ is granted to the anonymous user, anyone can get the list of Jenkins users.

As my user database (LDAP, or Active Direcotry) restricts the access to the list of users, I want to restrict also the access to People page.

This pull request contains following two changes:
1. Add a configuration to restrict users who can access People page in the top page:
    ![privilege_to_people](https://f.cloud.github.com/assets/3115961/1946223/c7909f28-7ff3-11e3-8324-6b85cba3c83a.png)
2. People page in the user-defined views get to display not committers of ALL projects in that view, but committers of READABLE projects in that view.