Privileges for Admin Control Panel

[julianlam]

• Introduces the admin privilege set, which joins the category privilege and global privilege sets
• Fixes a bug where giving or rescinding global privileges didn't fire the correct hook (breaking change)
• New hooks: action:privileges.admin.give, action:privileges.admin.rescind, fired when hooks are granted and rescinded, respectively.

I will append changes to the PR description as they are made.

[julianlam]

Breaking Changes

• When global privileges are granted, the action:privileges.global.give and action:privileges.global.rescind hooks are now fired. Prior to this change, action:privileges.categories.give and action:privileges.categories.rescind were incorrectly fired with cid 0 -- no cids are passed in the global variants of the hook
• If a theme overrides ACP pages, some templates (namely, partials/menu.tpl, partials/quick_actions/buttons.tpl, and admin/dashboard.tpl) will need updating