Break up long string into a heredoc

I know heredocs are weird, but this is *probably* more readable.

I always need to remind myself how the various flavours of heredoc
actually behave. Squiggly heredocs (<<~) remove indentation from their
content. It'll remove the same amount of indentation across all lines,
equal to the indentation of its least-indented line. In this case, all
of our lines start out indented equally, so all indentation ends up
removed from the resulting string.

Also worth noting: the replacement of newline chars in the old string
with actual line breaks in the heredoc. The tests pass, so I hope that
that means the output is identical in all cases that we care about.

lib/govspeak.rb

@@ -302,7 +302,12 @@ def render_image(image)
     end
 
     extension("address", surrounded_by("$A")) do |body|
-      %(\n<div class="address"><div class="adr org fn"><p markdown="1">\n#{body.sub("\n", '').gsub("\n", '<br />')}\n</p></div></div>\n)
+      <<~BODY
+
+        <div class="address"><div class="adr org fn"><p markdown="1">
+        #{body.sub("\n", '').gsub("\n", '<br />')}
+        </p></div></div>
+      BODY
     end
 
     extension("legislative list", /#{NEW_PARAGRAPH_LOOKBEHIND}\$LegislativeList\s*$(.*?)\$EndLegislativeList/m) do |body|