Allow crown's fallback image to have custom path

[peteryates]

The previously-hardcoded value makes a lot of assumptions on how the application is bundling its assets, this should offer a little more flexibility.

Refs #256

[peteryates]

This appears to be a deprecated attribute and it's blank - safe to drop it?

[frankieroberto]

I think this was deliberately included, as a work-around for some legacy browser bug.

But looking at the current HTML for the header in the Design System, it looks like it has been dropped, and the image has been wrapped in IE conditional comments now:

      <a href="#" class="govuk-header__link govuk-header__link--homepage">
        <span class="govuk-header__logotype">
          <!--[if gt IE 8]><!-->
          <svg aria-hidden="true" focusable="false" class="govuk-header__logotype-crown" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 132 97" height="30" width="36">
            <path fill="currentColor" fill-rule="evenodd" d="M25 30.2c3.5 1.5 7.7-.2 9.1-3.7 1.5-3.6-.2-7.8-3.9-9.2-3.6-1.4-7.6.3-9.1 3.9-1.4 3.5.3 7.5 3.9 9zM9 39.5c3.6 1.5 7.8-.2 9.2-3.7 1.5-3.6-.2-7.8-3.9-9.1-3.6-1.5-7.6.2-9.1 3.8-1.4 3.5.3 7.5 3.8 9zM4.4 57.2c3.5 1.5 7.7-.2 9.1-3.8 1.5-3.6-.2-7.7-3.9-9.1-3.5-1.5-7.6.3-9.1 3.8-1.4 3.5.3 7.6 3.9 9.1zm38.3-21.4c3.5 1.5 7.7-.2 9.1-3.8 1.5-3.6-.2-7.7-3.9-9.1-3.6-1.5-7.6.3-9.1 3.8-1.3 3.6.4 7.7 3.9 9.1zm64.4-5.6c-3.6 1.5-7.8-.2-9.1-3.7-1.5-3.6.2-7.8 3.8-9.2 3.6-1.4 7.7.3 9.2 3.9 1.3 3.5-.4 7.5-3.9 9zm15.9 9.3c-3.6 1.5-7.7-.2-9.1-3.7-1.5-3.6.2-7.8 3.7-9.1 3.6-1.5 7.7.2 9.2 3.8 1.5 3.5-.3 7.5-3.8 9zm4.7 17.7c-3.6 1.5-7.8-.2-9.2-3.8-1.5-3.6.2-7.7 3.9-9.1 3.6-1.5 7.7.3 9.2 3.8 1.3 3.5-.4 7.6-3.9 9.1zM89.3 35.8c-3.6 1.5-7.8-.2-9.2-3.8-1.4-3.6.2-7.7 3.9-9.1 3.6-1.5 7.7.3 9.2 3.8 1.4 3.6-.3 7.7-3.9 9.1zM69.7 17.7l8.9 4.7V9.3l-8.9 2.8c-.2-.3-.5-.6-.9-.9L72.4 0H59.6l3.5 11.2c-.3.3-.6.5-.9.9l-8.8-2.8v13.1l8.8-4.7c.3.3.6.7.9.9l-5 15.4v.1c-.2.8-.4 1.6-.4 2.4 0 4.1 3.1 7.5 7 8.1h.2c.3 0 .7.1 1 .1.4 0 .7 0 1-.1h.2c4-.6 7.1-4.1 7.1-8.1 0-.8-.1-1.7-.4-2.4V34l-5.1-15.4c.4-.2.7-.6 1-.9zM66 92.8c16.9 0 32.8 1.1 47.1 3.2 4-16.9 8.9-26.7 14-33.5l-9.6-3.4c1 4.9 1.1 7.2 0 10.2-1.5-1.4-3-4.3-4.2-8.7L108.6 76c2.8-2 5-3.2 7.5-3.3-4.4 9.4-10 11.9-13.6 11.2-4.3-.8-6.3-4.6-5.6-7.9 1-4.7 5.7-5.9 8-.5 4.3-8.7-3-11.4-7.6-8.8 7.1-7.2 7.9-13.5 2.1-21.1-8 6.1-8.1 12.3-4.5 20.8-4.7-5.4-12.1-2.5-9.5 6.2 3.4-5.2 7.9-2 7.2 3.1-.6 4.3-6.4 7.8-13.5 7.2-10.3-.9-10.9-8-11.2-13.8 2.5-.5 7.1 1.8 11 7.3L80.2 60c-4.1 4.4-8 5.3-12.3 5.4 1.4-4.4 8-11.6 8-11.6H55.5s6.4 7.2 7.9 11.6c-4.2-.1-8-1-12.3-5.4l1.4 16.4c3.9-5.5 8.5-7.7 10.9-7.3-.3 5.8-.9 12.8-11.1 13.8-7.2.6-12.9-2.9-13.5-7.2-.7-5 3.8-8.3 7.1-3.1 2.7-8.7-4.6-11.6-9.4-6.2 3.7-8.5 3.6-14.7-4.6-20.8-5.8 7.6-5 13.9 2.2 21.1-4.7-2.6-11.9.1-7.7 8.8 2.3-5.5 7.1-4.2 8.1.5.7 3.3-1.3 7.1-5.7 7.9-3.5.7-9-1.8-13.5-11.2 2.5.1 4.7 1.3 7.5 3.3l-4.7-15.4c-1.2 4.4-2.7 7.2-4.3 8.7-1.1-3-.9-5.3 0-10.2l-9.5 3.4c5 6.9 9.9 16.7 14 33.5 14.8-2.1 30.8-3.2 47.7-3.2z"></path>
          </svg>
          <!--<![endif]-->
          <!--[if IE 8]>
          <img src="/assets/images/govuk-logotype-crown.png" class="govuk-header__logotype-crown-fallback-image" width="36" height="32">
          <![endif]-->
          <span class="govuk-header__logotype-text">
            GOV.UK
          </span>
        </span>
      </a>

[frankieroberto]

Ah yes, this was changed back in May: Change approach to fallback PNG in the header to fix blank data URI from triggering CSP error

[frankieroberto]

I wonder if it'd be safest, for now, to not include the image at all if the URL isn't given? (As the fallback URL is quite likely to be wrong)

[peteryates]

Yeah - I agree. I've pushed another commit that removes it unless a crown_fallback_image is provided.

[frankieroberto]

Looks good. I think we just need to drop the xlink:href: attribute now.

Also maybe crown_fallback_image should be named crown_fallback_image_path just to be super explicit that we’re expecting a path to be set here?