Code formatting and documentation

* Remove unused $current_site global
* Add braces for if()
* Modify spacing
* Add docblock to admin_enqueue_scripts
Correct another case of missing braces.
Only show My Networks and My Sites in admin menu when appropriate

This modifies the previous logic a couple ways:

* If a user has been added to a network, but to 0 sites, do not show
  a menu that may indicate otherwise. This removes the My Sites and
  My Networks display.
* If a user is not a super admin, but is a member of more than one
  site, DO show the my networks menu. We'll get better at how that
  looks in the near future.
Handle menu for users who only belong to one network without admin

If a user is a member of only one network and they are not a super
admin of that network, do not remove the My Sites menu or show the
My Networks menu.
Only show a link to the network dashboard if the user is a super admin
Only show a link to Network Dashboard for capable users

Viewing the network dashboard requires manage_network capabilities
for that network.
Filter per network manage_network capabilities

This allows us to filter network administrators outside of the
is_super_admin check, which can only be relied on for global
admins.
Remove network admins from the global super admins list

The super admin role should be considered a global admin role only.
Capabilities should be used to handle permissions for users at the
individual network level.
Handle all manage_network capability checks

If any cap checks are done without specifying a network ID, we assume
they are for the current network.
Manage switching of networks through _wsuwp_switched_stack

Previously we piggybacked on _wp_switched_stack, which I can't imagine
was leading to the most consistent of results. By using our own stack,
we can better ensure multilevel network switching when doing various
tasks at the code level.
Check manage_network when determining class selector in menu

Replaces use of is_super_admin to create a border between network
dashboard and the list of sites.
Only show network admin toggle when is_network_admin

wp-admin/network/ only
Use network_admin rather than super_admin in the POST data

The POST data that isn't being sent to the server right now :/
Wrap network admin text with label to make it clickable
If network_admin is not POSTed, revoke super admin privileges
Only show sites a user has access to in their menu
Use a more appropriate manage_network check rather than super_admin

This will prevent sites that should show up from not showing up in
a user list for some networks.
Add a user to a network's main site when promoting to network admin
Apply our manage_network logic to all manage_network_* cap checks
Merge pull request #106 from washingtonstateuniversity/enhancement/networks-menu

[PR] Enhance the My Networks / My Sites menu around network permissions