Check for dependencies in entry point and Maintenance.php

Check for dependencies in entry point and Maintenance.php

This way, if someone tries to install MediaWiki via
either web installer or install.php maintenance script,
after obtaining from git, they get some useful information
on what to do.

Put the checks alongside the php version check, as
stuff installed via composer (mustache / lightncandy)
is used for NoLocalSettings.php and install.php
attempts to use logging stuff.

Also tried to make PHPVersionError look slightly nicer,
with some <h2> elements and more padding for the <p>
elements.

and centralized this code in one place, as much as possible,
for improved ease of maintenance.

Bug: T90438
Bug: T88951
Change-Id: Iae4eb42c4266dbe9213c5de8a96fccfbeaa9acb0