Web UI: fix "settings" button hidden in mobile view

[cnotin]

Before: settings icon is missing in mobile view

After: settings icon is back

Code is same as non-mobile CSS:

transmission/web/style/transmission/common.scss

Lines 805 to 808 in 2ef0751

	#settings_menu {
	@include roundedBox(5px);
	@include buttonImage('images/settings.png', $idle-color-top, $idle-color-bottom, $active-color-top, $active-color-bottom);
	}

[ckerr]

This is fixed by #1476.

Thank you for the PR though. This fix made sense when you submitted the PR and the maintainers (including me) should've reviewed your PR sooner. We'll try to do better in future.