Add license headers

r?

This adds MPL headers to all of servo's files.

Even though there are 9 non-moco contrtibutors to the main repo, `git blame` only indicates about a half-dozen lines of non-moco code here, none if it copyrightable in my non-expert opinion. So I've gone ahead and put the headers everywhere.

All that's left is rust-stb-image and rust-opengles, both of which have potentially non-trivial outside contributions.

I do have emails out to get permission from everybody. 6 have responded so far.  Once I've got all the responses I'll tar and sign them and throw them in the repo somewhere.