Fix status counter in old IE versions

We use flexbox to lay out the counts of sending/delivered/failed on the
activity and job pages. flexbox makes the best use of the space when
the numbers can be significantly different widths (eg 0 sending, 5000
delivered).

Flexbox was only supported from IE 11 onwards [1]. And since we were
setting the `display` property of the individual numbers to `block` they
were rendering one-per-line on browsers that don’t support flexbox.

This commit changes these items to be floated and a predefined width. In
browsers that support it, flexbox seems to override these hard-coded
widths. It’s not quite as good as the flexbox solution because:

- it doesn’t adjust the widths in the nice way that flexbox does

- it’s hard-coded to expect 4 items (we don’t have this component with
  any other number of items at the moment, so it won’t actually break
  anything)

But it’s pretty much OK because:

- it’s a lot better than the before

- IE 8 and 9 combined only make up 5% of our users, and this will be a
  declining number

- polyfilling flexbox would mean using Javascript, and we don’t serve
  working Javacript to IE 8 users anyway

1. http://caniuse.com/#feat=flexbox

app/assets/stylesheets/components/pill.scss

@@ -5,6 +5,9 @@
   a,
   span {
     display: block;
+    width: 25%;
+    box-sizing: border-box;
+    float: left;
     padding: 10px;
     flex-grow: 1;
     text-align: left;