Sharing: Handle narrow screens better.

When screens get narrow, the 'Share with:' label starts to wrap to two lines.
Since this is a translated string, we can't really just set a minimum width on
it and expect it to always work, but we can handle the wrapping case better.

This reduces the line-height to look less terrible, and keeps the labels
centered with the rest of the content when they need to wrap.

r2/r2/public/static/css/components/mixins.less

@@ -84,6 +84,11 @@
   display: flex;
 }
 
+.align-items(@props) {
+    -webkit-align-items: @props;
+    align-items: @props;
+}
+
 .clearfix() {
     &:before,
     &:after {

r2/r2/public/static/css/post-sharing.less

@@ -49,6 +49,7 @@
     display: block;
 
     .c-form-group {
+      .align-items(center);
       .display-flex();
 
       > * {
@@ -57,7 +58,6 @@
 
       > .post-sharing-label {
         .flex(0 0 20%);
-        line-height: @post-sharing-icon-size + (@margin-small * 1px);
         padding-right: @margin-small * 1px;
         text-align: right;
       }