Override default upload URL on all sites to force structure

We know that we won't be using ms-files to process files or storing
files in a directory structure other than year/month uploads.

This overwrites anything previously set by forcing sites to use the
sites portion of the uploads directory for storing content.

Definitely warrants further thought, but this will correct some
strange issues that have popped up recently
Merge pull request #159 from washingtonstateuniversity/fix/media-url

[PR] Override default upload URL on all sites to force structure