Support a small template system for changelog version descriptions

[erunion]

Can we make the field names in the description yellow like the body (eg in "Deprecated metadata.connections.activities in the User representation" metadata.connections.activities should be yellow)

We should add in a small template system for Mill changelog version descriptions so we can wrap key pieces of data in declarations that can then be converted into styleable HTML elements.

The format of the template should be something like {parameter}parent_folder{/parameter} or {representation}Video{/representation}. Follow the current set of supported elements we have in Mill changelogs: https://github.com/vimeo/mill/wiki/Generating-Changelogs#json

When converted into HTML within the JSON formatter, these should be wrapped in span tags akin to how it already works in changelog changesets:

<span class="mill-changelog_param" data-mill-parameter="parent_folder">parent_folder</span>
<span class="mill-changelog_representation" data-mill-parameter="representation">Video</span>

And in the Markdown formatter, they should just be wrapped in backticks like everything else.