Markdown: expose related markup as paired tags

Markdown contains multiple markup structures that are implicitly paired, but which we expose as two independent codes rather than a single pair.

The attached testcase contains two examples, a hyperlink:

This first testcase includes a <x id="1"/>link to the Okapi website<x id="2"/>, which should be exposed as a pair of tags.

and inline code backticks:

This second testcase includes <x id="1"/>code text<x id="2"/> in backticks.

Both of these would be more usefully exposed as paired codes.

Testcase attached.