Installer and upgrader rewrite

[Sesquipedalian]

@jdarwood007, let's use this to work collaboratively on the installer/upgrader rewrite.

• Add Schema for 3.0
• Add Initial framework for Maintenance logic
• Build Installer logic
• Setup Migration logic for upgrades from SMF 2.0 to 2.1
• Setup Migration logic for upgrades from SMF 2.1 to 3.0
• Test Installer logic
• Build Upgrader logic
• Test Upgrader logic
• Build Converter Logic
• Setup Converter logic for YabbSE
• Setup Converter logic for SMF 1.0
• Setup Converter logic for SMF 1.1
• Build Repair Settings tool
• Build logic for self-contained tools to utilize the maintenance logic.
• Add logic back for installer to work with compression (?obgz)
• Allow upgrader to support login or entering in the database password.

Fixes #8261

[jdarwood007]

A lot is incoming, it's my initial work I started to write the installer logic. You should be able to get through an install. But the base logic should get it going for an upgrader and converter logic.

[jdarwood007]

I've also added some task notes based on what I was thinking, please amend or adjust.

[Sesquipedalian]

Please remember not to force-push to a shared branch, @jdarwood007. I was able to reconcile the differences in my local repository fairly easily this time, but another time I might not be so lucky.

[jdarwood007]

@Sesquipedalian
I'm running into an error:
( ! ) Fatal error: Uncaught Error: Typed static property SMF\User::$me must not be accessed before initialization in /Sources/BrowserDetector.php on line 131

Its being called in the Migration/v2_1/BoardDescriptions

					'name' => Utils::htmlspecialchars(strip_tags(BBCodeParser::load()->unparse($row['name']))),

What is the optimal call here to put the best effort that SMF can do to safely startup the User::$me object? Keeping in mind we may be coming from a env in which it may not be able to startup things.

[jdarwood007]

		// Load up the current user safely.
		$p = User::load(0);
		User::$me = $p[0];

This seems to call ok.

[Sesquipedalian]

		// Load up the current user safely.
		$p = User::load(0);
		User::$me = $p[0];

This seems to call ok.

This would be better:

		// Load up the current user safely.
		User::setMe(0);

Also, is it indeed the intention here to load a guest user? Because that's what loading user 0 does.

[Sesquipedalian]

Moved to Alpha 4 as per the internal roadmap.

[Sesquipedalian]

Due to of a lot of ugliness and misery involved in trying to resolve conflicts, and due to @jdarwood007 informing me that he has limited time to work on it for the next while, I've decided to close this PR and reimplement this code in a new one. The fundamental logic is all there now thanks to @jdarwood007's work, so I will just need to make a few adjustments and finish off the remaining migration step classes, etc., that have not been written yet.