WSTEAM1-1029 - Form uploading screen

[amoore108]

Part of https://jira.dev.bbc.co.uk/browse/WSTEAM1-1029

Overall changes

• Adds 'Uploading' screen to UGC Form
• Adds initial screen switching switch/case to allow state updates to change the displayed screen
• Small change to background gradient to allow it to fill the entire screen regardless of form height
• Moves Form header and description elements down into the <Form /> component and out of the root
• Adds Storybook stories for the 2 current screens

Code changes

• Adds <Uploading /> screen component
• Exposes FormContext.Consumer to get access to the context at the root of the page to allow screen switching
• Removes the derivedHtmlType logic as this is now determined in the BFF
• Adds initialScreen prop for the root page to determine which screen to show initially. More so for Storybook benefits

Testing

1. Visit https://wsteam1-1029-form-uploading-screen--5d28eb3fe163f6002046d6fa.chromatic.com/?path=/story/pages-ugc-page--uploading
2. Confirm the layout meets UX and a11y requirements

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