Bug 1054178 - [Gaia][calendar] support 24 hour time format

I find it really weird that we need to check if it's 12 or 24 hour format and call exactly the same methods multiple times and build the same kind of element (`span` with `[data-date]` and `[data-l10n-date-format]`) on all the templates... we should abstract this logic better (maybe a new template file or a helper function shared by all the templates)