Force default og:image to https

If we can't figure out a good image to hint as a thumbnail for a page via
`og:image`, we set it to the reddit snoo icon.  However, we have been making
this a protocol-relative url.  This doesn't appear to be against [the spec][0],
but it does create problems for some scrapers.

Now we force it to be an https url, which should resolve some of those issues.

[0]: http://opengraphprotocol.org/#url

r2/r2/lib/pages/pages.py

@@ -1511,7 +1511,10 @@ class LinkInfoPage(Reddit):
             # thumbnails.
             return self.link.thumbnail
 
-        return static('icon.png')
+        # Default to the reddit icon if we've got nothing else.  Force it to be
+        # absolute because not all scrapers handle relative protocols or paths
+        # well.
+        return static('icon.png', absolute=True)
 
     def _build_og_description(self, meta_description):
         if self.link.selftext:

r2/r2/lib/template_helpers.py

@@ -65,6 +65,10 @@ def static(path, absolute=False, mangle_name=True):
     path_components = []
     actual_filename = None if mangle_name else filename
 
+    # If building an absolute url, default to https because we like it and the
+    # static server should support it.
+    scheme = 'https' if absolute else None
+
     if g.static_domain:
         domain = g.static_domain
     else:
@@ -93,7 +97,7 @@ def static(path, absolute=False, mangle_name=True):
         query = 'v=' + str(file_id)
 
     return urlparse.urlunsplit((
-        None,
+        scheme,
         domain,
         actual_path,
         query,