Add support for changing the colour of the introduction slide title, subtitle and "scroll to story" button

[RyanCoulsonCA]

Description

After #505 is merged, users will be able to add a background image to their introduction slide.

The metadata editor does not currently support changing the colour of the other introduction slide components such as the title, subtitle and "scroll to story" button, which will lead to WCAG contrast errors:

Proposed solution

The Storylines platform already supports changing the colour of these components and the properties do exist in the config. We just need to add some sort of colour picker to the metadata editor to allow changing these colours.