Additional Markdown/MathJax support

[ybnd]

https://github.com/DSpace/dspace-angular/pull/1851 adds a way to support Markdown/MathJax with the new dsMarkdown pipe. This pipe can be used anywhere we use [innerHtml], but the scope of that PR scope is limited to cases where rendering Markdown and/or MathJax doesn't pose additional difficulties.

The following use cases should be considered:

• Support dsMarkdown within ds-truncatable (e.g. in search result abstracts)
  • the height of rendered content is not guaranteed to fit within an integer number of lines, causing the content to be cut off
  • this could be addressed by adding a fadeout gradient along the bottom of the entire component
• For the titles of some mathematical works, headings and breadcrumbs should also allow for limited MathJax
  • only inline math should be allowed
  • only limited Markdown syntax should be allowed (e.g. text style, but no headings/lists)

Other suggestions are welcome, of course!