Add Fortnite and Twinmotion to shared credentials

[breakgimme]

Overall Checklist

• I agree to the project's Developer Certificate of Origin
• The top-level JSON objects are sorted alphabetically
• There are no open pull requests for the same update

for shared-credentials.json

• There's evidence the domains are currently related (SSL certificates, DNS entries, valid links between sites, legal documents etc.)
• If using shared, the new group serves login pages on each of the included domains, and those login pages accept accounts from the others. (For example, we wouldn't use a shared association from google.co.il to google.com, because google.co.il redirects to accounts.google.com for sign in.)
• If using from and to, the new group, the from domain(s) redirect to the to domain to log in.

Evidence:

• there already exists an identical case for unrealengine.com
• the websites link to each other via the menu in the top left corner
• logging in to any of the shared websites makes sso requests to the others