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.

tests/test_api/test_markdown.py

@@ -0,0 +1,53 @@
+import copy
+
+from pillar.tests import AbstractPillarTest
+from pillar.tests import common_test_data as ctd
+
+
+class CoerceMarkdownTest(AbstractPillarTest):
+    def test_node_description(self):
+        from pillar.markdown import markdown
+        pid, uid = self.create_project_with_admin(24 * 'a')
+        self.create_valid_auth_token(uid, 'token-a')
+        node = {
+            'node_type': 'group',
+            'name': 'Test group',
+            'description': '# Title\n\nThis is content.',
+            'properties': {},
+            'project': pid,
+            'user': uid,
+        }
+
+        created_data = self.post('/api/nodes', json=node, expected_status=201,
+                                 auth_token='token-a').json()
+        node_id = created_data['_id']
+
+        json_node = self.get(f'/api/nodes/{node_id}', auth_token='token-a').json()
+        self.assertEqual(markdown(node['description']), json_node['_description_html'])
+
+    def test_project_description(self):
+        from pillar.markdown import markdown
+        from pillar.api.utils import remove_private_keys
+
+        uid = self.create_user(24 * 'a', token='token-a')
+
+        # Go through Eve to create the project.
+        proj = {
+            **ctd.EXAMPLE_PROJECT,
+            'description': '# Title\n\nThis is content.',
+            'user': uid,
+        }
+        proj.pop('picture_header')
+        proj.pop('picture_square')
+        proj.pop('permissions')
+
+        r, _, _, status = self.app.post_internal('projects', remove_private_keys(proj))
+        self.assertEqual(201, status, f'failed because {r}')
+
+        pid = r['_id']
+
+        json_proj = self.get(f'/api/projects/{pid}', auth_token='token-a').json()
+        json_proj.pop('node_types', None)  # just to make it easier to print
+        import pprint
+        pprint.pprint(json_proj)
+        self.assertEqual(markdown(proj['description']), json_proj['_description_html'])