Category view modified to use column width

Category view modified to use column width

Category view used css media queries to set column count to 3 beyond 768px and
reduces the count to 1 below it. This behaviour has been changed by setting the
column width to 25em and allowing the column number to adjust automatically.
As a result, media queries have been removed.
See https://css-tricks.com/almanac/properties/c/column-width/
Follow up on https://gerrit.wikimedia.org/r/#/c/187952/

Bug: T55130
Change-Id: I20b68dea8f64b2d07866f4aa20ef83941c879697