Allow background images for active buttons

[MartijnCuppens]

Background images are removed from active buttons. This was initially done to remove the gradients in Bootstrap 3. In v4 this behaviour was moved to scss/mixins/_buttons.scss and only applied when gradients are enabled.

bootstrap/scss/mixins/_buttons.scss

Lines 41 to 43 in 29f178d

	@if $enable-gradients {
	background-image: none; // Remove the gradient for the pressed/active state
	}

This code was never removed in scss/_buttons.scss:

bootstrap/scss/_buttons.scss

Line 43 in 29f178d

background-image: none;