Add option to copy existing template when adding

Sometimes when setting up a service you might have a few very similar
templates, in which only a small amount of content. Or you might even
have a few of services, which are used by different teams but have
similar templates.

Copy and pasting, especially from one service to another, is a pain.
This commit makes it easier by allowing users to copy an existing
template when choosing to add a new one, instead of starting from
scratch.

app/assets/stylesheets/_grids.scss

@@ -81,6 +81,14 @@
   margin-bottom: $gutter * 2;
 }
 
+.left-gutter {
+  padding-left: $gutter;
+}
+
+.left-gutter-4-3 {
+  padding-left: $gutter * 4 / 3;
+}
+
 .align-with-heading {
   display: block;
   text-align: center;