Fixes srch-765
When navigating to `/cody/dashboard` and the new web app is enabled, the
user gets a "Not found" error. This doesn't happen in the React app.
The page doesn't exist in the new web app, hence the server should fall
back to the old web app but that doesn't happen.
Why? Because the server doesn't know about `/cody/dashboard` either. It
only exists in the React app.
Instead the server interprets this path as a (non-existing) repository
page. When the new web app is enabled, repository pages are handled by
the new web app, but since neither the repo nor the page exist in the
new web app, the aforementioned error is thrown.
Configuring this route on the server makes it so that the React app is
served instead.
## Test plan
Manual testing. Going to https://sourcegraph.test:3443/cody/dashboard
loads the React app.