Use 'pt-sandbox' rather than 'pt-mysandbox'

The latter was a pointless calque from the English Wikipedia. The 'my'
is vestigial, the link doesn't say "My sandbox" for a few years now.
Some gadgets on other wikis use 'pt-sandbox' already.

Change-Id: I04a6822594dd4751fb9e0e01f91f101fc5c88824