Update data entry form to allow editing of the human readable origin date text node (`//creation/origDate/text()`)

[wlpotter]

Sparked by #153. The cmproc:post-process-origDate function will be updated to ignore cases where there is already something in the text node. This will allow editors to specify alternatives to the '400 - 405 CE' date format, both at data creation and when revising

To facilitate this, the data entry form should allow editing of the human readable date.

This field should include instructions that make clear the following:

• the ISO-formatted date attributes need to be included as normal, e.g. '0400 0405'
• when creating new data, leaving the human readable field empty will result in a date of the form '400 - 405 CE'. The field should only be filled out if a different date string is preferred.
• when editing, if the date attributes are updated, you should check the human readable field to see if it is still accurate. Deleting the date string will allow the script to update the string to the normal date form, but with the new dates supplied in the attributes.

[wlpotter]

Another wrinkle to this. Currently, cmproc:post-process-origDate works by parsing the simplified date string (of the form 0400 0600, or -?\d{4}\s?-?\d{4}) into a human readable date, like "400 - 600 CE". In order to implement this data template change, the processor will have to pass through non-empty text nodes as-is. If the text node is empty, take the attribute values and parse them to create the human-readable date.

[wlpotter]

This has been updated in the css form to allow manual editing of the human readable date text node and the normalized attributes. I've also modified the processor to handle these changes but want to do some further testing before closing this issue.