form for setting MEDIA_BASE to arbitrary path

We've designed ddr-local to work with repos on external USB drives
but now we need to be able to connect to the local filesystem
as well.  This hack adds a small form at the bottom of the
/storage/ view that lets the user set an arbitrary filesystem
path as the target of the MEDIA_BASE symlink.
Merge branch 'master' of https://github.com/densho/ddr-local