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
Update Scientific American

Needs testing with an actual subscription
Edit metadata dialog: Allow editing identifiers in a dedicated window by right clicking on the identifiers edit box. Fixes #1400925 [[Enhancement] Improve the method for managing Indentifiers](https://bugs.launchpad.net/calibre/+bug/1400925)
Fix #1401085 ['calibredb list --for-machine' ignores custom columns](https://bugs.launchpad.net/calibre/+bug/1401085)