Tweaks:
- Never cache main page, always recalc book stats as needed.
- #466: strip whitespace from jinja comment.
- #497: allow .m4a audio files.
- #437: exclude unknown terms from language term counts.
Bug fixes:
- #455: handle TermTag associations when tags are deleted.
- #465: term import fix, only change statuses if explicitly specified.
- #458: omit empty error messages on language save.
- #464: prevent dictionary tabs from stealing focus. By @cblanken in #459.
- #293: fix stats chart shrinking. By @cblanken in #456.