finished 6.1, functions.php change, 6.1 python

I have finished the 6.1 assignment and made a few changes to the
6.1-multiplication.py script, including a rename to
6.1-multiplication-quiz.py. I think the assignment is free of
grammatical errors, as I have fixed all of those picked up by Word.

I have also changed functions.php, so that it no longer removes newlines
from the final HTML output. I have updated index.php so that is doesn't
replace newlines in code with "ESCAPED-NEWLINE" as well. I may
re-implement the removal of newlines at some point, but I will need to
work out a way of doing so that doesn't remove newlines inside of <pre>
or <code> elements.

In index.php, also started using is_readable() rather than file_exists(),
as the former performs the same test as the latter, while also ensuring
the file can be read. I have also slightly updated the error message
shown if this returns false.

Next up: 30.1. Fun!