Add redis database for directory SHA1s

Directory that maps SHA1 of directory name to the directory name.
This can be used for URL shortening while still not requiring persistent
state.
Use SHA1 for shorten directory IDs

While passing lists of directory names in GET requests, use the SHA1 of
the directory name rather than the directory name itself. This allows
for short directory labels and supports larger number of directories
in the request query string.

Currently used for "Show Expt Dirs" and comparison pages