Fixed height on .filter-group not needed on smaller screens

[BrashRebel]

The following CSS rule on line 551 of style.css:
`.filter-group {
...
height: 710px;
...
}`
...should be amended to apply only on screens wider than 1120px. On smaller screens the groups stack and it is no longer necessary to apply a fixed height to all.

[obenland]

In 1236:

WP.org Themes: Improve responsive styles for feature filters.

• Only applies a fixed hight when filter groups are not stacked. Props BrashRebel.
• Keps a top margin for the last filter group when stacked.
• Displays filters fluently to maximise hrizontal space on small screens.

Fixes #863.