Add an ‘add logo’ button

Because some people don’t know they can put their own logo on letters:

> The HM Government Logo is at the top of the letter and we can't see
> a way of putting the [organisation] logo on

> We are intending to use the letter template feature for the first time
> and wondered whether the branding is configurable or whether the HM
> Government header is the standard default.

> Can we replace HM Government logo with our own in the letter? IF yes,
> then how?

> I don't seem to be able to set the branding on the letters to be
> [organisation]. it's always HM government. Is there something that
> needs enabling for this account?

No-one actually wants the HM Government logo (no-one is sending real
letters using it). So we should leave the space blank and put a button
there prompting people to add their own logo.

app/assets/stylesheets/views/template.scss

@@ -45,3 +45,9 @@
   top: 51px; // aligns bottom edge to bottom of postmark
   right: 145px; // Aligns right edge to midpoint of postmark and fold
 }
+
+.edit-template-link-letter-branding {
+  @extend %edit-template-link;
+  top: 51px; // aligns with ‘change postage’ link
+  left: 66px; // Aligns to left of logo area
+}