Add customizations for the MS Sites List Table

Begins customizing the sites list table to provide more useful
information for WSU network administrators.

This gets us most of the way there. Will need to pick back up with
some cleanup and documentation.
Add documentation to our WSU Network Sites List class
Group column sets after unsets for more obvious organization
Remove the spam actions from the site list table

We likely won't require this at all.
Actually. Filter the actions list rather than deleting the actions.

Otherwise we're removing ourself too much from what happens in WordPress
core. I'd rather be able to adjust to any changes made there.
Display the site's URL under the site's name
Assign an empty string to spam/unspam rather than unsetting

This seems more appropriate, as defaults of empty strings are set
in the original array.
Show a user count rather than a list of users in sites table

This links to the site-users.php page so that users can be added
or edited quickly.
Document user count display
s/display_site_name_row/display_site_name
Cache the user count for display_site_users
Display the site URL in a different column
Use unset after all
Unset activation and archive actions from the sites list

We can add these if/when we want to use them.
Remove large chunk of unnecessary code.

I was trying to maintain consistency with core here, but it really
doesn't make sense if we're unsetting almost all of the default
actions that are setup.
Ensure user_count is an integer before displaying
Code braces for code style
Remove all bulk actions on the sites list table.

These are not needed for our purposes right now and we should try
to avoid the accidental bulk removal of a bunch of things.
Merge pull request #63 from washingtonstateuniversity/sites

[PR] Add customizations for the MS Sites List Table