Merge pull request #243 from tagawa/small-edits

Added links to Twitter and mailing list
Better message for missing issues from untriaged.
Different caches for staging and production.
Let's switch to if/else.
Forward etag and cache-control (and set proper mime) for /issues/lables.

Issue #239. and Issue #241.
proxy_request should return the Response object, like GitHub-Flask does.
Forward caching headers for /api/issues/number.

Issue #239 & #241.
Get rid of ensure_xhr decorator (mostly useless, IMO).
Add caching headers to the homepage "browse" endpoints.

Issues #239 & #241.
Fix posting label changes.
Return the correct mimetype and add cache headers for the /comments endpoints. Issue #239 #241.
Return proper mimetype and response status when an issue is
opened/closed. #241.
Remove obsolete test.
Test the new "browse issues" sections on the homepage.
same class
fixed height label editor fullscreen
to prevent zoom on iOS
Switch to animate instead of just scrollTop.
Fix unclosed span in footer.
Remove spam comment + doctypes for svg files.