Fix inconsistent behaviour between image syntaxes

There was inconsistent behaviour between the different syntaxes used for
embedding images.

The `!!n` syntax must start on a new line. But the previous line can
contain other content (e.g. paragraph text).

For example, this is valid:

```
Some example content, immediately followed by an image.
!!1
```

Whereas the `[Image:]` syntax had to be preceded by two newline
characters (i.e. a new Markdown paragraph).

For example:

```
This does not work:
[Image: example.jpg]

But this does because it's preceded by two newline characters:

[Image: example.jpg]
```

This inconsistent behaviour was spotted by publishers after we
implemented support for the `[Image:]` syntax in Whitehall Publisher,
since they're used to the `!!n` syntax which is more lenient.

We've decided to bring the two in line by adjusting the `[Image:]`
syntax so that it only needs to be preceded by one newline. This still
avoids the issue identified in 663e099
because it does not allow images to be embedded within a paragraph.

I've tweaked the `[Image:]` regex and adding extra tests to cover the
desired behaviour. I've also added equivalent coverage to the `!!n`
syntax tests to explicitly call out this behaviour and ensure the two
syntaxes behave consistently.

lib/govspeak.rb

@@ -424,7 +424,7 @@ def self.devolved_options
       renderer.render(contact: ContactPresenter.new(contact))
     end
 
-    extension("Image", /#{NEW_PARAGRAPH_LOOKBEHIND}\[Image:\s*(.*?)\s*\]/) do |image_id|
+    extension("Image", /^\[Image:\s*(.*?)\s*\]/) do |image_id|
       image = images.detect { |c| c.is_a?(Hash) && c[:id] == image_id }
       next "" unless image
 

test/govspeak_images_bang_test.rb

@@ -80,4 +80,30 @@ def initialize(attrs = {})
       )
     end
   end
+
+  test "!!n syntax must start on a new line" do
+    given_govspeak "some text !!1", images: [Image.new] do
+      assert_html_output("<p>some text !!1</p>")
+    end
+
+    given_govspeak "!!1", images: [Image.new] do
+      assert_html_output(
+        "<figure class=\"image embedded\"><div class=\"img\"><img src=\"http://example.com/image.jpg\" alt=\"my alt\"></div></figure>",
+      )
+    end
+
+    given_govspeak "!!1 some text", images: [Image.new] do
+      assert_html_output(
+        "<figure class=\"image embedded\"><div class=\"img\"><img src=\"http://example.com/image.jpg\" alt=\"my alt\"></div></figure>\n<p>some text</p>",
+      )
+    end
+
+    given_govspeak "some text\n!!1\nsome more text", images: [Image.new] do
+      assert_html_output <<~HTML
+        <p>some text</p>
+        <figure class="image embedded"><div class="img"><img src="http://example.com/image.jpg" alt="my alt"></div></figure>
+        <p>some more text</p>
+      HTML
+    end
+  end
 end

test/govspeak_images_test.rb

@@ -88,5 +88,13 @@ def build_image(attrs = {})
         "<figure class=\"image embedded\"><div class=\"img\"><img src=\"http://example.com/image.jpg\" alt=\"my alt\"></div></figure>\n<p>some text</p>",
       )
     end
+
+    given_govspeak "some text\n[Image:image-id]\nsome more text", images: [build_image] do
+      assert_html_output <<~HTML
+        <p>some text</p>
+        <figure class="image embedded"><div class="img"><img src="http://example.com/image.jpg" alt="my alt"></div></figure>
+        <p>some more text</p>
+      HTML
+    end
   end
 end