Fixing loading behaviour according to the documentation

[FIX] ms-ds-consistencyguid need also to be Base64 encode

Fix count slave checking

The use of count function is wrong and unnecessary. If the array is
empty the foreach loop will not run.

Updated config.php template to include comma after baseURL

Fixes https://github.com/simplesamlphp/simplesamlphp/issues/818

* Check responseHeaders presence before use them

And so avoid some messages in log file.

This is minor, but if a comma is required when this is enabled the template should reflect that.

Document the new Twig templating system

It is deprecated as of PHP 7.2

Edits to CONTRIBUTING.md

This marker is used by some Markdown compilers to start the Table of
Contents hierarchy at this point.

Per suggestion from @tvdijen

Add some basic documentation on how the new Twig templating system works.

PSR-2 compliant whitespace in error classes

space is required before auth data

forward-port of my recent fix in branch 1.15

Fixes issue https://github.com/simplesamlphp/simplesamlphp/issues/793

It was already added under 1.15

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This section felt a little like a blob of good, unorganized thoughts. I
hope this is a little easier to read and understand. It will be easier
for someone to go to this document and point to a specific section now.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This is a general readability improvement. The meaning of these
paragraphs has not changed.

This comment is quite arbitrary in the view of the general public. Not
sure if anyone would know what the acronym "T.O.C." means.