Fix asset precompilation

[mamhoff]

What is this pull request for?

Fix asset precompilation in host apps with SassC in environments other than development. We were still using the old image-url Sass directive, whereas modern Sass/Sprockets can fingerprint assets referenced in URL directives directly.

Notable changes (remove if none)

Makes the current main branch deployable.

Screenshots

None :)

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.47%. Comparing base (a13577a) to head (15a79fd). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3024 +/- ## ======================================= Coverage 96.47% 96.47% ======================================= Files 237 237 Lines 6404 6404 ======================================= Hits 6178 6178 Misses 226 226 ```

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