Create Organisation logo component

[fofr]

Create a component for "medium sized" "stacked" logos, ie those used on HTML publications.

• Port styles from static’s existing organisation styles, but tweak to match Whitehall exactly
• Use the newer GDS-Logo stack for organisation logos
• Use the new frontend toolkit list of colours to set border colour
• Allow logos with and without links
• If a crest is needed it needs to be passed in explicitly
• The organisation name should be pre-formatted
• Special case executive office branding (eg Prime Minister's office)

To help with building this I created a quick page in Whitehall to render all the medium stacked logos for every organisation:
https://github.com/alphagov/whitehall/compare/org-logos

Tests will fail until alphagov/govuk_frontend_toolkit#262 has been merged and this can be version bumped.

Example usage:

<%= render partial: 'govuk_component/organisation_logo', locals: {
  organisation: {
    name: "Attorney <br>General’s <br>Office",
    url: "/government/organisations/attorney-generals-office",
    brand: "attorney-generals-office",
    crest: "single-identity"
  }
} %>