[frontend] broken redirect to `/submissions`

[alxndrsn]

Repro

1. open https://staging.getodk.cloud/projects/203/forms/trees_update/
2. observe "Page Not Found" Error:

Additionally

The page header indicates that the user is "Not logged in", even when they are. Filed at #1698.

Cause

https://staging.getodk.cloud/projects/203/forms/trees_update/ seems to append /submissions to the URL. This works OK if the original URL had no trailing slash.