Add columns option to Special:PrefixIndex

Adds support for specifying the number of output columns to use in the
table generated by Special:PrefixIndex. Can be used as follows:

{{Special:PrefixIndex/{{FULLPAGENAME}}/|columns=2}}

Bug: 60759
Change-Id: Ic824351132ec8bf7c35116f6ab19f23386ead33e
Merge "Add columns option to Special:PrefixIndex"