Use mdx_linkify on all the fields where we're using markdown

Fixes PMT #96043

I realized that we're not always saving markdown-generated HTML to the
database. This adds the 'linkify' extension wherever we use the markdown
template tag. Action item descriptions, user bios, etc should all have
correct link tags now.
Merge pull request #116 from nikolas/linkify-fields

Use mdx_linkify on all the fields where we're using markdown