Remove unsupported metadata fields upon first edit

We should remove `meta:document-statistic` and `meta:editing-duration` upon the first edit, because otherwise they'd be inconsistent with the rest of the document and with `dc:date` respectively.

These two are not being dealt with right now because:
* `meta:editing-duration` deals with ISO durations which are not trivially possible to reliably make without a good library (think 28 days in February, leap years, etc).
* `meta:document-statistic` is a different/new feature that requires some significant time investment.