fix(api-ext): use same save logic as the api

[fmartingr]

• Modified the extension API to use the same save flow as the regular API, by generating the bookmark first and trying to archive it later. This will result in a stored bookmark even if the archiving fails.
• Switched the session priority, now the header has more priority than the cookie, witch should be the default, since the webserver sends the cookie and the extension requires the session anyway.
• Fixed the DB.GetBookmark method by handling the sql.ErrNoRows silently (since it already exposes a bool).

Fixes #517