Increase maximum file extension length from 3 to 15 characters

For Viewable, Testable, Compressible file types and download events,
the file extension is no longer limited to 3 characters. 15 characters
seemed like "enough".

Similarly, a user's temp/QWK file type preference is also extended from
3 to 15 characters.

This fixes issue #486, but file types such as *.tar.gz (2 dots in the extension)
are still an issue and will be addressed next.