Outline style to be configurable.

[alan-cole]

User story

As a theming developer
I want define the style of outline used for focus indicators
So that I can match established branding for that website

Description

While outline width and outline offset are configurable variables, the outline-style is not.

Further to this, the outline-style used in input fields is not using the ct-outline() mixin, but is re-defined.

This means that updating the style from solid to anything else requires multiple overrides.

There should be a $ct-outline-style: solid !default; variable defined in variables.components.scss, and used throughout for focus indicators.

Acceptance criteria

Given I am a theming developer
When I add $ct-outline-style: dotted; in variables.components.scss
Then I should see all focus indicators use the dotted style.

[alan-cole]

Further to this,

1. The ct-button doesn't define the outline-width: $ct-outline-width; on it's focus indicator.
2. .ct-menu__item__link-trigger doesn't set a ct-outline()
3. .ct-logo doesn't set a ct-outline()
4. .ct-tag doesn't set a ct-outline()
5. .ct-back-to-top__button should not have an outline-color: transparent. It should use $ct-outline-light.