Mark all strings in templates translatable

Review can be automated using https://github.com/rory/django-template-i18n-lint 

It doesn't do recursive search, so

    find . -name '*.html' -o -name '*.txt' | xargs -I {} ./django-template-i18n-lint.py {}

does the trick. It finds quite a few false positives, but there's still at least a dozen unmarked strings in there.