Allow any user input data to be located anywhere within data folder

[carlhiggs]

Currently there is flexibility for some but not all user inputs as to where they are located within the data folder.

Specifically, transit feeds are currently required to be in the 'data/transit_feeds' folder, and report images are in the 'configuration/assets' folder. The latter in particular is a bit obscure for users and really just a legacy from when we developed our initial 25 city report series.

We should allow users to locate their resources anywhere in the data folder, either using the provided sub-folders or their own region specific folder --- including transit feeds and report images.