DLTP-186: Render formatted text attributes

Run plain-text attributes through a Markdown processor and sanitize the
output before it is inserted into a template.

First pass at implementation using RDiscount

Protect against script injection

Make quotation marks fancy

Remove untested class method

Moving utility classes to lib

Text parsing helper method and placeholder test

Adjusting rendering of abstracts; placeholder test

Tidying up tile formatting

Token test for #richly_formatted_text helper

Tests for #curation_concern_attribute_to_html

Allow setting a custom class for descriptive text blocks

Updating expected markup for edit view test

Use #public_send in lieu of #send

Adding back ETD abstract attribute rendering