Made it possible to customise the placeholder image

[iainbeeston]

Description

Makes it possible to replace the avocado image used for placeholders with a custom image.

Fixes #1243

Checklist:

• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works

Manual review steps

1. Set the placeholder value in the branding config with the path to an image
2. Create a new resource that has a grid but don't set a cover image
3. Check that the new placeholder image is used
4. Remove the placeholder value from the branding and restart
5. Confirm that the placeholder changes back to an avocado.

Manual reviewer: please leave a comment with output from the test if that's the case.

[qlty-cloud-legacy[bot]]

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[adrianthedev]

I pushed some changes to make it work with our current assets. I keep the avocado.svg asset because we use it somewhere else in the app.

Thanks for your contribution!