Make Datepicker translatable

Summary: This adds the Ablillity to translate the Content of the Date Picker. It is possible to display more than one Letter for the Day Captions.

Test Plan:

• Change the Language in the User Preferences, make sure it translates 'S|M|T|W|T|F|S' and Months or add the following to the translation.override config to get German Translation:

{
    'S|M|T|W|T|F|S': 'So|Mo|Di|Mi|Do|Fr|Sa',
    'January': 'Januar',
    'February': 'Februar',
    'March': 'März',
    'May': 'Mai',
    'June': 'Juni',
    'July': 'Juli',
    'October': 'Oktober',
    'December': 'Dezember'
}

• Open Datepicker on a Datefield (Edit Event or Edit for Custom Field of Type date)
• The Day Headers and the Months should now be translated

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: CSharp, 0, speck, tobiaswiese

Tags: #calendar

Differential Revision: https://we.phorge.it/D25016