Small improvement on template tester messages.
Enhancement #1360766: show plugboards for disabled devices and allow entering plugboards when no metadata writer is enabled.
Preferences->Metadata plugboards: Show plugboards for disabled devices and formats whose metadata writer plugins have been disabled. Fixes #1360766 [display plugboards with disabled plugin](https://bugs.launchpad.net/calibre/+bug/1360766)

Merge branch 'master' of https://github.com/cbhaley/calibre