refactor(ui): Simplify json option templating

[PythonGermany]

Summary

Changes how template placeholders for complex types are replaced. Not sure if something like this has been proposed before.

For some other features I'm working on I struggled for a while getting template placeholder substitution to work for nested map options. This PR is the solution I came up with to make it easier to do that.

Tested it in local environment with this config:

ui:
  header: "Custom"
  logo: "https://github.githubassets.com/favicons/favicon.svg"
  link: "https://github.com"
  dashboard-heading: "Custom Dashbard"
  dashboard-subheading: "Custom subheading for the dashboard"
  default-filter-by: "failing"
  default-sort-by: "name"
  buttons:
    - name: Quad9
      link: "https://quad9.net"
    - name: DuckDuckGo
      link: "https://duckduckgo.com"

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• (n/a) Updated documentation in README.md, if applicable.