Modify the default upload filetype list - include ppt, remove zip

We'll create the concept of "extended permissions" where a site can
apply to have access to upload exe, dmg, and zip files.
Provide a base for extending a site's allowed filetypes list

Some sites do not need the ability to upload dmg, exe, or zip files

In fact, exe files are a beast of their own that we may not want
to allow in the end. We'll have a trigger available that turns a
site into one with extended file permissions. This will upgrade the
list of allowed filetypes and the amount of space per file allowed
on a per site basis.
Remove exe from the list of allowed downloads
Provide an extension for the max file upload size

By default, this will up to 100MB for sites that have been given
extended permissions
Display option to apply extended permissions to a site on the network
Save a request to apply or deny extended permissions to a site
Allow csv, keynote, numbers, and pages uploads at the basic level
Super admins (global admins) have the ability to upload exe

I'm still not entirely sure how I feel about this, but we can start
by restricting exe uploads to those with super admin access, which
is really a small handful.

Once we figure out what automated virus scanning could look like, this
could be implemented at a wider level
Remind WP of the exe mime type after it has removed it
Reapply the removed exe mimetype
Reduce filter duplicity, handle mime types in one area
Merge pull request #156 from washingtonstateuniversity/file-types

[PR] Introduce "extended" site permissions to handle multiple levels of site options