Custom DateTime format to allow loading of formats

[mt3571]

Fixes #1121, #1138 and #646 (See #1121 for discussion)

This fix means that the format string is auto-populated with DD/MM/YYYY HH:mm:ss but the type is set to Custom. This means that if the user shares the recipe via a URL the correct format with be saved and used.

[mattnotmax]

Would be nice to get this merged and closed out if possible thanks!