Lead users who need to register to install, back to installation after account registration and activation

In order for users to install the "multi-server edition", they need to login.  If they haven't registered (and it's unlikely they have), they need to register first... which also requires activation.  By the time they activate, they'll have to restart the installation process, which can be confusing ( #917 )

Now, we expose a "next" URL parameter for the link to "sign up" on the login page.  When this is used, the user will actually be redirected to their desired page upon successful registration and activation.

Testing:
1. Click "install"
2. Click "multi-server edition"
3. From the login page, click "sign up"
4. Register, and copy the activation link (should contain a "next" URL parameter)
5. When activating, you should be brought to the login page (should contain a "next" URL parameter).
6. After logging in, you should be brought to the "multi-server edition" install page.

@wangguan59 could you try this out?