Add pace tools that import pace tools

Yo dawg...

Adds pace tools that allow superadmins to import pace tools from a local
file or an external URL. It only imports them into the editor of the
client who ran them.

(The superadmin restriction is present to deter people from stealing
other's pac costumes. It's obviously still possible, but I don't want
this to make it any easier to do. In singleplayer the local player is a
superadmin so they can still use it there.)

See the following gist for some example importable pace tools:

https://gist.github.com/suchipi/632a3bfb8a29269c1ddb