Implement the Tudor crown in the Header component (v4.x)

[querkmachine]

Implement the Tudor crown iconography in the Header component. Part of #4179.

See #4297 for the equivalent changes for Frontend v5.

Warning

This PR should not be merged until we have approval to do so.

Changes

• Adds the Tudor crown SVG to the Header component.
  • Both crowns (and their fallbacks) have been moved out into their own private variables to try and keep the bulk of the HTML easy to read.
• Adds a new Tudor crown fallback graphic for IE8.
• Adds a new Nunjucks parameter to the Header component, useTudorCrown, to switch to using the new Tudor crown.
• Adds some styles adjusting the positioning of the Tudor crown, if it's in use.
• Adds tests ensuring that the Tudor crown is being rendered if the flag is set, and repeats the same tests carried out for the St. Edward's crown.

Todo

• When complete, this PR can (probably) be wholesale copied for v3.x with little to no alteration.

[owenatgov]

Looks good to me! I ummed and ahhed about the width="32" targeting over using a modifier class but it's probably the simplest way to do it considering we'd ask users to change that value anyway, and IMO the impact of a user missing setting width to 32 if they're not using our macros is pretty low.

We should be explicit in our release notes that users not using our macro should set the width when updating the svg, or ideally just re-copy a new svg.

[querkmachine]

Merging in preparation for release w/c 15th January.