MimeTypeController generates a map of file extensions to MIME types

It uses `mime.types` file from apache http located under
http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/conf/mime.types?view=markup

This file has been placed in the public domain for unlimited
redistribution,
so we can use it and ship it with Yii.

see also yiisoft/yii#2851 and yiisoft/yii#2857

fixes #3255

controller links application vendor to yiidev repo

[ci skip]

issue #3026

adjust existing Console Controllers and docs

fixes #2184

fixed Markdown::process() to make it compatible with parent declaration.

Control statements based on the if and elseif constructs must have a single space before the opening parenthesis of the conditional and a single space after the closing parenthesis.