Add multipart/form-data emulation to ve.init.mw.Target#apiRequest

On my example main page, this reduces the size of the POST body by 45%

Bug: 59660
Change-Id: I7f04f403e0081188b8b4024c4431ae43b127434c
Merge "Add multipart/form-data emulation to ve.init.mw.Target#apiRequest"