Cache Markdown'ed HTML in database

This is done via coercion rules. To cache the field 'content' in the
database, include this in your Eve schema:

    {'content': {'type': 'string', 'coerce': 'markdown'},
     '_content_html': {'type': 'string'}}

The `_content_html` field will be filled automatically when saving the
document via Eve.

To display the cached HTML, and fall back to display-time rendering if it
is not there, use `{{ document | markdowned('content') }}` in your template.

Still needs unit testing, a CLI command for regenerating the caches, and
a CLI command for migrating the node type definitions in existing projects.

pillar/markdown.py

@@ -47,3 +47,11 @@ def markdown(s):
                              attributes=ALLOWED_ATTRIBUTES,
                              styles=ALLOWED_STYLES)
     return safe_html
+
+
+def cache_field_name(field_name: str) -> str:
+    """Return the field name containing the cached HTML.
+
+    See ValidateCustomFields._normalize_coerce_markdown().
+    """
+    return f'_{field_name}_html'