Create admin during installation, allow multiple admins

[qwerty287]

Requires #1539 to be merged.

• create the admin user during installation
• add command to create new users
• allow multiple admins (in theory, untested)

TODO

• add installation step to create user
• add command to create user
• remove related legacy code
• fix code and tests to not longer assert that admin is user with ID 0
• fix tests (does not find tables users and sym_links !?)
• increment all user IDs (no longer use ID 0 to fix Laravel issues)
• Add command to reset the password of a user (useful if you are locked out of admin)
• remove old code
• Password confirmation check on admin creation.

[ildyria]

At initial admin user creation, there need to be a password validation check, namely a second password field to confirm that the password is entered properly as chosen (to avoid typos mistakes etc).

Ideally there needs to be a visual feedback that the button has been clicked.
See in: resources/views/update/error.blade.php (does not need to be the same but similar)

	<!-- Do not change even a single character in the block below without
	     also updating the checksum in config/secure-headers.php! -->
	<script>
	document.addEventListener("DOMContentLoaded", function(event) {
		document.querySelector("form").addEventListener("submit", function(e){
			document.querySelector("form").hidden = true;
			var text = document.createElement("div");
			text.innerHTML = "Migration started. <b>DO NOT REFRESH THE PAGE</b>.";
			document.querySelector(".form").appendChild(text);
			// e.preventDefault();    //stop form from submitting
		});
	});
	</script>

[qwerty287]

Ideally there needs to be a visual feedback that the button has been clicked.
See in: resources/views/update/error.blade.php (does not need to be the same but similar)

I'd do this in another PR and get this one out first,

[ildyria]

LGTM.