Add directories to promdash

Adds directories to clean up dashboard organization a bit

requested in #52 

This pr is getting kind of weighty (already ~500 lines). The original issue also asked for adding "owner" emails to dashboards and last accessed times, with potentially regular reminders going out to owners of dashboards that hadn't been accessed in a while. Would you like to move that feature to a separate pr? There were some other things as well — filtering, potentially drag and drop re-ordering (I would like to see how many dashboards end up in a directory before implementing that), etc.

I would also like to, in the future, add in-line dashboard creation from within a directory grouping. That would probably entail moving the index to be managed by angular, which i think would also make this pr unnecessarily large.

Thoughts?