DRY data files

[igneus]

We have a lot of files sharing the same calendar data and only differing in localized feast names. Let's make it possible to load a calendar from two files, one containing the shared structure (date, rank, colour, symbol) and another one mapping symbols to feast names.

Note: at first it may look like a good idea to move sanctorale feast names to i18n - especially when temporale feast names are already there. (And pejulian/romcal goes more or less this way, too.) But this would make it impossible or very complicated to work with two calendars, using the same locale and having different names for the same feast. This is often the case when one of the calendars is diocesan and the other religious.