[#8604] update file upload widget

[hom3mad3]

Update styles for the file uploader
This PR updates the styles for the file uploader with some compromises due to my lack of knowledge on the Django/Python side. I wasn’t sure how to create a new widget for the user-facing side, so I adjusted the styles on the existing snippet.

Changes:

• Style update
• Added the thumbnail preview inside the input, for better visual feedback. We should also indicate the file size and do client-side validation, but that also required too much django.
• Included file size display information (image_uploader.js: add file size information adhocracy4#1751)
• Removed the blue info box, as it wasn’t the right element for this input field and replaced it with a form-hint/help-text approach for better alignment with the design. I'd remove this text altogether, as it's confusing due to the different contexts.

Before upload:

With preview:

Select to delete image:

Side effects on dashboard help-text:

TODO (backend) - Would require more Django integration, which I couldn’t implement:

• We should probably separate the file uploaders or unify them into one design, everything is too tangled in django classes, so i don't know how to do that.
• The image rights-of-use checkbox should ideally be linked to the file uploader for better UX and validation.

[hom3mad3]

@CarolingerSeilchenspringer @mcastro-lqd tagging you for feedback on the design