Explain trial mode on the settings page

The previous text on this page around trial mode was a bit of a
mouthful. Also it only really made sense if you already knew what trial
mode was.

This commit tries to make it really explicit:
- that you’re in trial mode
- what it means to be in trial mode (copied from the trial mode page)
- where you go to not be in trial mode

app/assets/stylesheets/_grids.scss

@@ -35,6 +35,10 @@
   margin-bottom: $gutter-half;
 }
 
+.bottom-gutter-2 {
+  margin-bottom: $gutter * 2;
+}
+
 .align-with-heading {
   display: block;
   text-align: center;