Extract mixin to add a gap to a flexbox layout

This way we remove duplication and it'll be easier to add better support
for RTL languages.

In a few years this might not be necessary since support for the `gap`
property in a flexbox layout will improve. At the time of writing,
however, only 86.5% of the browsers support it [1].

[1] https://caniuse.com/flexbox-gap

app/assets/stylesheets/budgets/executions/filters.scss

@@ -2,14 +2,8 @@
   margin-bottom: $line-height / 2;
 
   @include breakpoint(medium) {
-    $gap: 1em;
+    @include flex-with-gap(1em);
     align-items: flex-end;
-    display: flex;
-    margin-left: -$gap;
-
-    > * {
-      margin-left: $gap;
-    }
   }
 
   select {