(bug 34852) new optional 'link' parameter to <gallery>

(bug 34852) new optional 'link' parameter to <gallery>

The patch adds an optional parameter |link= to the <gallery>
tag. This will allow for images to link to other pages and
externals urls instead of being hardlinked to the image file
that is displayed in the gallery.

Here are a couple of examples.

Link as WikiLink:
<gallery>
File:20120106_001.jpg|link=Main_Page
</gallery>

Link as absolute URI:
<gallery>
File:20120106_001.jpg|my caption|alt=my alt
text|link=http://bugzilla.wikimedia.org
</gallery>

this would cause the link on the thumbnails rendered by the gallery tag to link
to a custom page/url instead of the actual media/image.

a link should be an internal wiki link or an absolute uri as shown in the examples.

Change-Id: I21b276ad5c7a8df13b3a716957d23fd53c37d29e