batchdelete: decode location.pathname before using it as gapprefix

Would cause double percent-encoding and apinvalidtitle.
batchdelete: use decodeURIComponent instead for decoding pathname

Pages like [[E=mc^2]] would fail with decodeURI.
Merge pull request #169 from jimmyxu/for-azatoth

batchdelete: decode location.pathname before using it as gapprefix