add abbreviated form of liturgical rank

[JohnRDOrazio]

Currently we only include the following properties in a calendar event, as regards liturgical rank:

• grade: integer representing the liturgical rank from 0 to 7
• grade_lcl: string representing a localized form of the liturgical rank
• display_grade: string representing a localized form of the liturgical rank that should be used in place of grade_lcl if non empty

We do not however include any abbreviated form, which can be useful for displaying a liturgical calendar.

We could add yet another property grade_abbr with localized forms of abbreviations.

For example in English:

• S	= Solemnity
• F✝	= Feast of the Lord
• F	= Feast
• M	= Memorial
• m	= Optional Memorial
• m*	= Optional Commemoration
• w	= weekday

In Italian:

• S	= Solennità
• F✝	= Festa del Signore
• F	= Festa
• M	= Memoria
• mf	= Memoria facoltativa
• mf*	= Commemorazione
• f	= giorno feriale

In Latin:

• S	= Solemnitas
• F✝	= Festum Domini
• F	= Festum
• M	= Memoria obligatoria
• m	= memoria ad libitum
• m*	= commemoratio
• f	= feria

Perhaps we will also need a display_grade_abbr property? Only two cases so far are:

• All Souls Day: has a grade of Solemnity, but displays as Commemoration (perhaps shouldn't display anything? Commemoration is perhaps only meant to show in the name?)
• Dedication of the Lateran Basilica: has a grade of Feast of the Lord, but displays as Feast

[JohnRDOrazio]

Implementing this will allow to continue with Liturgical-Calendar/liturgy-components-php/issues/7

[JohnRDOrazio]

related issue: #253