Task-add: Use a button instead of a link, and prevent default

Avoids the issue of re-loading the page on task-add

src/styles/_app_base.sass

@@ -206,18 +206,6 @@ nav.sidebar
 						&.active
 							background-image: none
 
-					&.task-add
-						width: auto
-						border-radius: initial
-						opacity: 0
-						vertical-align: middle
-						padding: 5px
-						display: flex
-						justify-content: center
-
-						&:hover, &:focus
-							opacity: 1
-
 					&:hover
 						&:before
 							opacity: .8
@@ -225,6 +213,22 @@ nav.sidebar
 						&.active:before
 							transform: translate(-50%, -50%) scale(1.3)
 
+				button.task-add
+					display: flex
+					width: auto
+					border-radius: initial
+					opacity: 0
+					vertical-align: middle
+					padding: 5px
+					justify-content: center
+
+					color: $color-primary
+					border: none
+					background: none
+
+					&:hover, &:focus
+						opacity: 1
+
 .item-list-header
 	a
 		color: $color-text-dark