Modify `makemessages.py` to add the keyword for i18n for handlebars files.
Undo the changes on `makemessages.py` and modify `trans_real.py` instead because using `djangojs` will not work because the .handlebars is re-formatted by `prepare_js_for_gettext()` like javascript.  The handlebars templates are actually similar to django templates except for the tag delimiters used.
Include the handlebars extension only for `djangojs` domain but process it like a Django template.
Update DJANGO-CHANGES.txt by adding diff in: `makemessages.py` so that we can identify what we changed.
Add diff in `trans_real.py` so that we can identify what we changed.
Merge branch 'develop' into feature/2472-have-makemessages-scan-handlebars-files-to-extract-translatable-strings
Merge branch 'develop' into feature/2472-have-makemessages-scan-handlebars-files-to-extract-translatable-strings
Update the i18n file with sample instructions.
Merge pull request #2698 from mrpau/feature/2472-have-makemessages-scan-handlebars-files-to-extract-translatable-strings

Modify `makemessages.py` to add the keyword for i18n for handlebars files.