Import: add compatibility for milliseconds in NETSCAPE-Bookmark

NETSCAPE-Bookmark sometimes contains dates as milliseconds instead of seconds.
For instance, this is the case for the files generated for Google +1s by Google Takeout.
This patch make these files compatible.

Example of such a file generated from Google +1s:

```html
<!DOCTYPE NETSCAPE-Bookmark-file-1>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=UTF-8">
<TITLE>Bookmarks</TITLE>
<H1>Bookmarks</H1>
<DL><p>
<DT><H3>Google +1s</H3>
<DL><p>
    <DT><A HREF="http://www.engadget.com/2013/06/17/airbus-a350-cockpit-video-tour/" ADD_DATE="1376683982550" LAST_MODIFIED="1376683982550">Airbus A350 cockpit tour with test pilot Jean-Michel Roy (video)</A>
</DL><p>
```