Move instance-level sections of Dagit to /instance paths.
- /assets/... -> /instance/assets/...
- /scheduler -> /instance/scheduler
- /runs?... -> /instance/runs?...
I also created a FallthroughRoot to perform redirects for the existing routes, in case of any paths that I missed converting, or in case users have bookmarks. In the future, these should probably be cleaned up.
There are a couple other paths to move to the /instance path space as well, but this gets things moving.