Adding capacity to use MarkdownExtra in page description field. Will exp...

...and to other fields in Admin.

The composer package used allows for [MarkdownExtra](https://michelf.ca/projects/php-markdown/extra/), which enhances what you can already do with Markdown, such as adding ids/classes to headers, definition lists and more! 

This PR only adds Markdown to the description text in static pages. Next steps include:

- add Markdown to static page blocks
- review what other fields might need markdown syntax

Since a new package has been included to composer, make sure to run `composer update`!

@angaither 

CC: @barryclark @DFurnes