(Vert.) center align work package filter flexboxes

baseline implementation appears to be buggy in Chrome.

Signed-off-by: Alex Coles <alex@alexbcoles.com>

app/assets/stylesheets/content/_work_packages_filters.sass

@@ -49,7 +49,7 @@ $work-packages-filters--inset-background-color: darken($work-packages-filters--b
 
 .work-packages-filters--filter
   @include grid-block(6)
-  align-items:      baseline
+  align-items:      center
   background:       #efefef
   padding:          0.25rem 0
   border:           1px solid #ccc
@@ -79,7 +79,7 @@ $work-packages-filters--inset-background-color: darken($work-packages-filters--b
 .work-packages-filters--add-filter
   @include grid-block(6)
   @extend %work-packages-filters--filter-part
-  align-items:      baseline
+  align-items:      center
   padding:          0.25rem 0
 
 .work-packages-filters--add-filter-label