bugdown: Blacklist linkifying the .md TLD.

Fixes #2065.

zerver/fixtures/bugdown-data.json

@@ -410,6 +410,11 @@
       "<p>%s</p>",
       "t.co"
     ],
+    [
+      "readme.md",
+      "<p>readme.md</p>",
+      "readme.md"
+    ],
     [
       "go to views.org please",
       "<p>go to %s please</p>",

zerver/lib/bugdown/__init__.py

@@ -71,8 +71,8 @@ def unescape(s):
 
 def list_of_tlds():
     # type: () -> List[Text]
-    # HACK we manually blacklist .py
-    blacklist = [u'PY\n', ]
+    # HACK we manually blacklist a few domains
+    blacklist = [u'PY\n', u"MD\n"]
 
     # tlds-alpha-by-domain.txt comes from http://data.iana.org/TLD/tlds-alpha-by-domain.txt
     tlds_file = os.path.join(os.path.dirname(__file__), 'tlds-alpha-by-domain.txt')