add js/css assets only on warmup pages

[staabm]

without this change, the warmup.(css|js) is included in all backend pages, so e.g. even on structure/ or similar.

while working at it I am wondering whether the styles are required at all. @schuer are you sure this files (and their contents) are still used?

Relates to https://github.com/redaxo/redaxo/issues/1406

[schuer]

Thx @staabm. I think you’re totally right and nobody’s noticed the issue yet.

However, I cannot test your warmup PRs right now but will do a redaxo session next week something. Thx in advance!

[staabm]

👍here we go

[staabm]

🤓

[schuer]

🎉 thx!