Add missing Danish holidays

[albertospelta]

The current version of the Danish holidays definition does not include three holidays.
Below are the details from sql-bi/Bravo#793 (comment)

• Palmesøndag (the Sunday before Skærtorsdag (Mandy Thursday)
• Påskedag (the Sunday before Anden påskedag (Easter Monday)
• Pinsedag (the Sunday before Anden pinseday)

[jkoinnovatedk]

• Palmesøndag (Palm Sunday) is by definition actually the last Sunday before Easter so it can be calculated by subtracting 7 from the date for the Easter Sunday (Påskedag)
• Påskedag (Easter) is the first Sunday after the first full moom after March 21th
• Pinse (Sunday of the feast of Pentecost) is the 7th Sunday after Easter