Support images in labels

[garethbowen]

Description

Provide support for adding images to labels.

The groundwork for this was done in #347 including the xforms-engine work to parse them.
The next step was actually rendering them which is being merged as part of #62 but this is just the most basic version.

During that we realised it needed some design work for alignment and size constraints etc, as discussed in this comment: #511 (comment)

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Checklist

• Does it need UI/UX design?
  • Yes. Describe the UI/UX requirements or attach a Figma link in the "Design" section.
  • No
• Does it need API design?
  • Yes. Specify the endpoints, methods, or data structures needed.
  • No
• Does it need design around state flow?
  • Yes. Outline the state flow requirements.
  • No
• Does it need a test plan?
  • Yes. Add the test plan as a comment in this task or attach the document.
  • No

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User Stories

• As a form designer,
  I want to add labels with images
  so that data collectors can have a visual reference when filling in forms.