New maintenance script for refreshing image metadata (refreshImageMetadata.php)

New maintenance script for refreshing image metadata (refreshImageMetadata.php)

This is very similar to rebuildImages.php, except more specific to img_metadata field,
and does the images in batches instead of all at once.

Also, while I'm here, I added a line to Maintenance.php to make sure it casted
$this->mBatchSize to an integer when gotten from command line (thought it was weird
that it didn't do that)

(I'm going to tag this revision 1.18 because I think it'd be nice to have this script
in 1.18 given new image metadata stuff added in 1.18, but not super-important
because rebuildImages.php does already work to refresh image metadata)