Major revision of comment system.

- Comments are stored in HTML as well as Markdown, so that conversion
  only happens when saving (rather than when viewing).
- Added 'markdown' Jinja filter for easy development. This is quite
  a heavy filter, so it shouldn't be used (much) in production.
- Added CLI command to update schemas on existing node types.

requirements.txt

@@ -5,6 +5,7 @@ algoliasearch==1.8.0
 bcrypt==2.0.0
 blinker==1.4
 bugsnag==2.3.1
+bleach==1.4.3
 Cerberus==0.9.2
 Eve==0.6.3
 Events==0.2.1
@@ -19,6 +20,7 @@ google-apitools==0.4.11
 httplib2==0.9.2
 idna==2.0
 MarkupSafe==0.23
+markdown==2.6.7
 ndg-httpsclient==0.4.0
 Pillow==2.8.1
 pycparser==2.14
@@ -48,6 +50,7 @@ cookies==2.2.1
 cryptography==1.3.1
 enum34==1.1.3
 funcsigs==1.0.1
+html5lib==0.9999999
 googleapis-common-protos==1.1.0
 ipaddress==1.0.16
 itsdangerous==0.24