Start managing network capabilities for users.

When a user is added through a network, we assume they have some
sort of capability on this network. By adding a meta key to track
this, we can better manage things in the future.
Add capabilities for the global platform to new users

If this user is being added to a network for the first time, they
should also receive basic capabilities at the global level.
Use add_user_meta for network capabilities to avoid overwrite

This shouldn't be possible, but it's worth being explicit about
intent here. Capabilties for this network should only be added if
no capabilities currently exist.
Ensure an absolute int before adding network capabilities

Even though wsuwp_get_current_network is pulling from the
current site global, it's worth avoiding notices if something
strange was to happen.
Add hooks for network and global capabilities when editing users

This will aid in getting current users assigned proper capabilities
before things get too out of hand.
Manage super admins through the $super_admins global

This disables the default core functionality for determining who
is a super admin and provides a custom array of users instead.

By doing this, we grab more granual control over the process. We
also lose the ability to assign super admins on the fly. This is
a good thing, as we can start to create that experience around
the concept of a global admin.
Introduce a get_global_admins method

Retrieves a list of current global admins, should have a more
public facing function at some point.
Introduce is_global_admin method

Determine if a user is a global admin
Make use of the new get_global_admins when setting super admins
s/super_admins/network_admins/ - replaced incorrect array
Display an option to assign super admin privs to a user.

This is only shown to global admins and replaces the previous
functionality for standard super admins.
Handle the toggle of super admin for a user

Adds methods for revoking and granting super admin privileges at
a network level. Those privs assigned when on the global network
will translate to global admin privileges.
Merge pull request #103 from washingtonstateuniversity/feature/network-caps

[PR] Introduce custom global and network capabilities