Persist date elements of a project as epoch date

[zfi]

Data elements of the Project object are persisted to storage a localDate string, epoch date format or simply "NaN". This request is an attempt to standardize the storage format of dates in the Project file. Going forward, all date elements should be stored as epoch dates.

There is currently code in Solo that is able to address the variety of existing date formats.

[zfi]

The project metadata is stored within the SVG footer element. The timestamps are currently persisted as human-readable date-time text but the text is not displayed in the watermark because it is translated to an area below the watermark display box.

It seems that all that is needed is to convert the timestamp strings to a text representation of the epoch value. The project loader is already aware of this date format and should load it without any issues.