Create markdown directive for Pub title field

[gabestein]

Motivation

So users can add the pub's title to a form without needing to know which field it is

Requirements

• Renders the pub's canonical/selected title field for the pub type as html in markdown editors
• Allows users to select the title from a parent, like any other field selection

Acceptance Criteria

• Uses helper from #736
• Uses format :value{field=title} or optionally :value{field=title rel=parent} (??)
• Should render the field's context as text, stripping any formatting if the field is encoded in markdown or html (the ideal is probably to preserve <em> or <strong> but to strip anything else, but we can do that later)

Optional Work

• If the field contains markdown or HTML, strips all elements except <em> or <strong>

[gabestein]

Just guessed on the estimate, so please correct.

Also, totally not sold on :value{title} as the directive and welcome suggestions.