Serve kepubs with extension "kepub.epub"

Change the extension of kepuns to "kepub.epub" so that the browser on
the Kobo ereaders will download them from the server. This checks the
useragent so that only Kobo browser is affected.

Fixes #1400029 [Serve kepubs with extension "kepub.epub"](https://bugs.launchpad.net/calibre/+bug/1400029)
Small performance improvements for the content server kepub patch