Added the network REST endpoint.

This adds support for past and future versions of WordPress, and may be removed or changed at a later date.

See: https://core.trac.wordpress.org/ticket/46964#comment:42

Networks List Table: correct condition that tests whether there are any network_admins for a network. (#151)

If our nonce field doesn't exist, bail. Otherwise we'll try to verify our nonce everywhere, which breaks all other forms.

Fixes #138

* Only check nonces on our forms
* Prefer `empty()` to avoid debug notices.

* Fix undefined variable: network 
* Updates comment to reference $network instead of $item

E.g. `esc_html__( 'Sites: <strong>%s</strong>', 'wp-multi-network' )` outputs `<strong>216</strong>` as text string and not as HTML: https://codex.wordpress.org/Function_Reference/esc_html

Fixes Warning: array_map(): Expected parameter 2 to be an array, bool given

* Introduce networks capabilities class, use capability checks instead of is_global_admin() and remove is_global_admin() function.
* Fix typo in admin bar class name.
* Display admin bar links to network screens based on available capabilities.
* Add list_networks to global capabilities.
* Fix incorrect network admin bar dashboard node identifier that was a duplicate before.
* Use manage_network capability check instead of is_super_admin() when validating a site name.
* Correctly use tabs instead of spaces for indenting.

This has been wrong for a while, and is probably my fault. It's supposed to kick in when rewriting is not in use, which is since 3.7.

Also some surrounding code clean-up to make ternary operators more clear.