Bug 1895594: Update/Remove external userid value when copying page

On the course completion block.

Otherwise the copier sees the content for the person they copied the
page from rather than their own content (or no content if they don't
have an external userid)

Change-Id: I2433df88b2462171b4644d9841e1bb2baa27509a
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>