Adding additonal path to search for git information on sysadmin dashboard

When viewing the sysadmin dashboard courses tag, we also search the git
repository directory in addition to the DATA_DIR to try and find
the course git repo to get hash, last commit, and last editor information
for each loaded course if available.

Suggested reviewers: @singingwolfboy and @cpennington since you did the original Sysadmin Dashboard review.
Heads up to @sarina.