start the studio app from a subfolder will always jump back to the root

[newgene]

Steps to reproduce:

1. goto https://studio.biothings.io/master
2. "Create a new connection" to connect to a hub
3. the webapp jumps back to / now

The webapp should stay under /master, when it connects to a hub.

we have multiple versions hosted at https://studio.biothings.io/:

• https://studio.biothings.io/ -- the current prod
• https://studio.biothings.io/master -- always synced with the master branch
• https://studio.biothings.io/0.1f -- an older version 0.1f (can be any older versions)

[newgene]

same as: https://github.com/biothings/biothings.api/issues/98