Riot Web expects the email validation next_link to have the sid appended

[jryans]

When we previously used Sydent to validate email at registration and redirect to the client, Sydent would append the sid value to the next_link so the client will have this without storing any state.

Now that Synapse sends email validation itself, we need it to do the same thing as well. Without this, it prevent Riot Web from continuing through the registration flow.

[jryans]

Fixed by #6097