Add readonly option on input and allow component to be used in a reactive from with an empty date

[tplessis]

Allow component to be used in a reactive form with an empty date without having an "Invalid date" message and with days of the current month.

Add a readonly option to calendar's input.

[CLAassistant]

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[jkuri]

thanks for you PR. can you sign the CLA please so I can merge this?