Support creation and update dates

[ArthurHoaro]

For POST and PUT link endpoint, the REST API will support new flags created and updated. They're dates in ISO 8601 format.

Related to:

• possibility to set created and updated dates from API/shaarli-client Shaarli#1223
• REST API: allow override of creation and update dates Shaarli#1525
• Add creation and update dates to LinkRequest api-documentation#36