Add transitions for content block addition and removal

assets/css/src/newsletter_editor/contentBlocks/base.styl

@@ -30,3 +30,8 @@ $block-hover-highlight-color = $primary-active-color
 
 .mailpoet_content
   position: relative
+
+.mailpoet_block_transition_in
+  animation-fade-in-and-scale-up()
+.mailpoet_block_transition_out
+  animation-fade-out-and-scale-down()

assets/css/src/newsletter_editor/contentBlocks/container.styl

@@ -79,3 +79,4 @@ $three-column-width = ($newsletter-width / 3) - (2 * $column-margin)
   box-shadow(inset 1px 2px 1px $primary-inactive-color)
   color: #656565
   border-radius(3px)
+  animation-background-color()

assets/css/src/newsletter_editor/mixins/transitions.styl

@@ -7,3 +7,45 @@ animation-slide-open-downwards()
     max-height: 0
     opacity: 0
     overflow-y: hidden
+
+animation-background-color()
+  transition: background 300ms cubic-bezier(0.420, 0.000, 0.580, 1.000) /* ease-in-out */
+
+animation-fade-in-and-scale-up()
+  animation-name: fadeInAndScaleUp
+  animation-duration: 500ms
+  animation-fill-mode: forwards
+
+animation-fade-out-and-scale-down()
+  animation-name: fadeOutAndScaleDown
+  animation-duration: 500ms
+  animation-fill-mode: forwards
+
+
+@keyframes fadeInAndScaleUp {
+  0% {
+    opacity: 0.3
+    max-height: 0
+    overflow: hidden
+  }
+
+  100% {
+    opacity: 1
+    max-height: 5000px
+    overflow: hidden
+  }
+}
+
+@keyframes fadeOutAndScaleDown {
+  0% {
+    opacity: 1
+    max-height: 5000px
+    overflow: hidden
+  }
+
+  100% {
+    opacity: 0.3
+    max-height: 0
+    overflow: hidden
+  }
+}