Book details panel: Fix custom columns built from other columns with tag like values not clickable in the book details panel. Fixes #1381323 [Custom Column build from other columns does not show up as clickable](https://bugs.launchpad.net/calibre/+bug/1381323)

Merge branch 'master' of https://github.com/cbhaley/calibre
AZW3 Output: Ignore invalid attribute names in the input document rather than aborting the conversion on them.