Add optional message to define default description for Upload

Adds message 'upload-default-description' (default to disabled)
that allows people to put in a default preload text for the
upload form (like commons does with js). Message is not displayed
if wpForReUpload url parameter is present (aka they clicked
upload a new version from an image description page). [It will
however be present if someone had manually made a url with
wpUploadDestName filled in with an existing file, but the
wpForReUpload was not present].

Bug: 14116
Change-Id: Ibc13da6114232c6870a8a907e7c4c60fad142353
Merge "Add optional message to define default description for Upload"