Use mixins for file thumbails

assets/sass/_variables.sass

@@ -60,25 +60,3 @@ $board-task-limit-color: #DF5353
 
 @function button-hover-border-color($key)
   @return map-get($button-hover-border-colors, $key)
-
-=appearance
-  -webkit-appearance: none
-  -moz-appearance: none
-
-=display-flex
-  display: -webkit-flex
-  display: flex
-
-=flex-direction($direction)
-  -webkit-flex-direction: $direction
-  flex-direction: $direction
-
-=flex-wrap
-  -webkit-flex-wrap: wrap
-  flex-wrap: wrap
-
-=align-content($position)
-  -webkit-align-items: $position
-  align-items: $position
-  -webkit-justify-content: $position
-  justify-content: $position