refactor: Reduce complexity of `TemplateLayout` constructor

[susnux]

Summary

This re-structures the constructor to be easier to understand, by splitting things into different helper functions.

TODO

• [ ] ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)