Oauth: make it so that you get redirected to the proper page after login

[petethepig]

Currently if a user opens a pyroscope link (e.g /?query=cart.service.dotnet.cpu%7B%7D) and they are not authenticated, we redirect them to oauth authentication flow. After they're done we redirect them to /, thus losing the original URL they were trying to open. We should preserve that original URL and redirect to it after the user is done with the login flow.

[petethepig]

I'm thinking maybe when we do the redirect we could preserve the URL in a cookie (or localstorage) and then after login we could check if this cookie exists and redirect.

I think another important thing with this feature is tests — ideally a cypress test to confirm that this stuff works, because I imagine we might change the authentication flow in the future.