Let users download a CSV of inbound messages

In user research, we’ve seen users copy/pasting the contents of the
inbound SMS page into a spreadsheet, in order to keep a record of the
messages they receive. They even went as far as to write a macro which
fixed the errors caused by copying and pasting.

It would be much easier if we just gave them the data already in a
spreadsheet format. Which is what this commit does.

One caveat is that, because spreadsheets can contain executable code (ie
formulas), and because we’re populating the spreadsheet with
user-submitted data (albeit via SMS) we need to be careful about
injection attacks.

The details of how these attacks work are detailed here (interesting
reading): http://georgemauer.net/2017/10/07/csv-injection.html

The mitigation is to not allow characters which initialise a formula
at the start of the cell.

app/assets/stylesheets/_grids.scss

@@ -39,6 +39,11 @@
   margin-top: $gutter * 4 / 3;
 }
 
+.top-gutter-2-3 {
+  @extend %top-gutter;
+  margin-top: $gutter-half;
+}
+
 %bottom-gutter,
 .bottom-gutter {
   @extend %contain-floats;