Make translator comment prefix case-insensitive

Translator comments currently must start with exactly "L10n:". Change so that's case-insensitive, e.g. "L10N:" or "l10n:" or "l10N:" also work.

This is for https://bugzilla.mozilla.org/show_bug.cgi?id=868178

(Note: Babel has recently released new versions that break the tower tests. I pinned the Babel version to 0.9.6 rather than trying to fix that in this branch.)