Enable analytics only while authenticated

The one place where we should log out of PostHog and reset our analytics ID is when the user is logging out. This matches the behavior in Element Web and makes sense, I think, because logging out is essentially a request for the app to forget who you are. This means we should also start analytics at the point of logging in / reauthenticating.

I noticed while making this change that there was an unused branch in setClient, so I cleaned it up rather than making myself update it.

src/auth/useRegisterPasswordlessUser.ts

@@ -47,7 +47,7 @@ export function useRegisterPasswordlessUser(): UseRegisterPasswordlessUserType {
           recaptchaResponse,
           true,
         );
-        setClient({ client, session });
+        setClient(client, session);
       } catch (e) {
         reset();
         throw e;