Have external-link component use the icon component rather than hard-coding the svg

[kruncher]

To workaround some sort of spacing issue the external-link component has the icon hard-coded as an svg directly into the template:

{# Icon needed to be directly put here instead of calling macro to solve an issue with extra space on the end of links being underlined #}

Instead the icon component should be used and adjusted as needed so that it works in this situation.

Benefits:

• Better quality unit test.
• Avoid duplication of svg data in generated bundle.
• One source of truth for the "external-link" icon.

[kruncher]

The macro tests should be updated to use the new templateFaker().spy('icons') functionality (see macro tests in button for an example on how to do this).

[yatesn]

This issue needs to be solved first #2873