Add frameless option to image component

[anandamaryon1]

Description

Our image component adds a visible border-bottom and a background colour by default. This option removes those so that images can be added more flexibly and to dark backgrounds, for example, in our hero component.

Current:

Frameless:

Checklist

• Tested against our testing policy (Resolution, Browser & Accessibility)
• Follows our coding standards and style guide
• CHANGELOG entry

[colinrotherham]

Since the default is the card background colour, this should be a string

But like other components, overriding this with background: false would remove it

[anandamaryon1]

Ah ok, will this mean we can add other colours in future, eg. page-background or NHS blue?

[colinrotherham]

Yeah if necessary, but also because we do this in the code component already 😊

[colinrotherham]

Maybe something like this?

Suggested change

	'If set to `false`, sets the background colour to transparent for the image container.',
	'Background colour for the image component – `"card"` or `false`. Defaults to `"card"`. To remove the background colour, set `background` to `false`.',

[colinrotherham]

We support both classes and border: true

So we'll need to check for both here, i.e. to skip the class if already set

Suggested change

	{%- if params.border === false %}
	{%- if params.border === false and (not params.classes or not "nhsuk-image--no-border" in params.classes) %}

[colinrotherham]

Suggested change

	{%- if params.background === false %}
	{%- if params.background === false and (not params.classes or not "nhsuk-image--no-background" in params.classes) %}

[anandamaryon1]

Thanks for the review @colinrotherham I've had a go at amending based on your comments.