animate New Issue box on Boards

I disabled animating the slide-in for the New Issue card on Boards. Best (easiest) way to animate multiple items is to wrap both the cards and the New Issue form in a transition-group

But transition group does't work with v-show (source). Changing the <ul class="board-list"> to use v-if means we Sortable.create needs to happen after issues are fetched, instead of when component is mounted.

The following discussion from !2467 (merged) should be addressed:

• @iamphill started a discussion: (+1 comment)

  New issue box jumps when its getting closed. Probably hard to see in the gif.