[3.0] Jumping to later installer steps is possible without a prior SMF installation having begun

[MisterVector]

In an environment with just a blank database, it is possible to go to a URL, such as: http://fork.development.local/smf/install.php?step=4

The installer will then start to install the forum without any prompts from previous steps.

[Sesquipedalian]

Yeah, we should end all use of URL parameters in the installer and upgrader, and instead rely entirely on the stored data that we write to Settings.php during the process.