Allow date_added to be specified in POST /api/bookmarks/ API

[jmason]

Hi!

I'm working on syncing my Pinboard bookmarks to a Linkding server, and have run into a minor issue. My Pinboard bookmarks have an existing timestamp and I'd like to preserve that, but the POST /api/bookmarks/ API does not allow date_added or date_modified to be set.

I'm working around this by using the Netscape bookmarks importer and an intermediate HTML file, but that's suboptimal.

Would you be open to a PR to made those 2 fields non-read-only? Cheers :)