Change parameters order to avoid YUI Compressor bug (#1680)

The previous code lead to an invalid value for the left property:

`left: calc(-50vw+50%);`

This was being caused due to a bug in YUI Compressor. You can read
a detailed explanation about it in this issue:

yui/yuicompressor#59


This is fixed since version 2.4.9-BSI-2. But, until we don't update
prod's YUI Compressor, this PR work as a bugfix since it changes the
parameters order to one properly understood by the bugged version.
More aboute this in this issue comment:

yui/yuicompressor#59 (comment)

Author: berinhard

static/sass/style.css

@@ -3661,7 +3661,7 @@ span.highlighted {
     display: block;
     width: 100vw;
     position: relative;
-    left: calc(-50vw + 50%); }
+    left: calc(50% - 50vw); }
     #sponsorship_application_container #benefits_container .title {
       margin: 0 0 1em 0;
       padding: 0;

static/sass/style.scss

@@ -2444,7 +2444,7 @@ span.highlighted {
         display: block;
         width: 100vw;
         position: relative;
-        left: calc(-50vw + 50%);
+        left: calc(50% - 50vw);
 
         .title {
             margin: 0 0 1em 0;