Add ?data= parameter

[cmdcolin]

This allows users to specify ?data=test_data to load test_data/config.json

This was proposed previously but stopped to let perfect be the enemy of good...maybe just let a little imperfect in #3192

[cmdcolin]

(the imperfect being the use of trailing slash to join the config.json onto the ?data= param)

[cmdcolin]

The term ?data= is the same URL param that jbrowse 1 used to load a "data directory" of files, loading data/trackList.json by default