Greatly improve the usability of the rebuild.php script

Updated: Comment #32

Problem/Motivation

Since there are many circumstances under which users may need to do this, we should make it super easy to do.

My experience today was:

1) I got Fatal error: Call to a member function get() on a non-object in /Users/webchick/Sites/8.x/core/lib/Drupal.php on line 241 when git pulling.
2) I thought to myself "Hm. I wonder if that newfangeldy rebuild.php script can fix that?"
3) I found the script in core/rebuild.php (this took awhile... I expected it in the root directory along with index.php and update.php)
4) I went to http://8x.localhost/core/rebuild.php.
5) I was redirected back to index.php (I only caught this because I was being extra observant) and still got the same error.
6) I did it again to make sure I wasn't nuts.
7) I checked README.txt for "rebuild" and found nothing.
8) I pinged chx on IRC who said it's a settings.php thing I need to set to TRUE. No idea how I would've discovered that otherwise, and "ping chx on IRC" is not a scalable solution to this issue. :D

Compare this with hitting update.php as an anonymous user:

It tells me what went wrong and exactly what I need to do to fix it.

Proposed resolution

Can we please make rebuild.php work in a similar way?

Remaining tasks

• (novice) change maintenance_access_denied_page() to be a theme function or a twig template. See #28
• open issue to refactor Kernel things. See #26
• open issue to refactor the minimal environment in install_begin_request() into a stand-alone function. See #31
• open issue for cleaning up default.settings.php

User interface changes

API changes

?