Fix some Markdown warnings

Fixes the following warnings when run with Markdown 2.6.11:

DeprecationWarning: "safe_mode" is deprecated in Python-Markdown. Use an HTML sanitizer (like Bleach https://bleach.readthedocs.io/) if you are parsing untrusted markdown text. See the 2.6 release notes for more info
DeprecationWarning: Using short names for Markdown's builtin extensions is deprecated. Use the full path to the extension with Python's dot notation (eg: "markdown.extensions.extra" instead of "extra"). The current behavior will raise an error in version 2.7. See the Release Notes for Python-Markdown version 2.6 for more info.

Also get rid of some compatability code for Markdown 2.0, which we no longer support.

Updates #257

pokedex/db/markdown.py

@@ -16,12 +16,8 @@ import re
 import markdown
 import six
 from sqlalchemy.orm.session import object_session
-try:
-    # Markdown 2.1+
-    from markdown.util import etree, AtomicString
-except ImportError:
-    # Old Markdown
-    from markdown import etree, AtomicString
+from markdown.util import etree, AtomicString
+
 
 @six.python_2_unicode_compatible
 class MarkdownString(object):
@@ -66,8 +62,7 @@ class MarkdownString(object):
             extension = self.session.markdown_extension
 
         md = markdown.Markdown(
-            extensions=['extra', extension],
-            safe_mode='escape',
+            extensions=['markdown.extensions.extra', EscapeHtml(), extension],
             output_format='xhtml1',
         )
 
@@ -228,6 +223,17 @@ class PokedexLinkPattern(markdown.inlinepatterns.Pattern):
             el.text = AtomicString(label or name)
         return el
 
+class EscapeHtml(markdown.Extension):
+    u"""Markdown extension which escapes raw html elements.
+
+    This is the recommended replacement for safe_mode='escape',
+    which was deprecated in Markdown 2.5.
+    See https://python-markdown.github.io/change_log/release-2.5/
+    """
+    def extendMarkdown(self, md, md_globals):
+        del md.preprocessors['html_block']
+        del md.inlinePatterns['html']
+
 class PokedexLinkExtension(markdown.Extension):
     u"""Markdown extension that translates the syntax used in effect text: