Add alt text fields on impact report page

[helenb]

Link to Ticket

Description of Changes Made

• Add alt text field and 'is decorative' field to impact report page hero image
• Add alt text field to instagram gallery image block. It isn't possible to add the 'is decorative' option here, as the alt text provides the text for the link.

How to Test

Edit an impact report page. Test adding some custom alt text for the hero image, and also try ticking the box to say that it is decorative.

Then test editing some images in the instagram gallery block. Test adding some custom alt text.

Screenshots

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MR Checklist

• Add a description of your pull request and instructions for the reviewer to verify your work.
• If your pull request is for a specific ticket, link to it in the description.
• Stay on point and keep it small so the merge request can be easily reviewed.
• Tests and linting passes.

Unit tests

• Added
• Not required

Documentation

• Updated build docs
• Updated editor guidelines (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only)
• Updated field spec (See https://intranet.torchbox.com/torchbox-com-project-docs for the private link, for Torchbox employees only)
• Not required

Browser testing

• I have tested in the following browsers and environments (edit the list as required)
  • Latest version of Chrome on mac
• Not required

Data protection

• Not relevant
• This adds new sources of PII and documents it and modifies Birdbath processors accordingly

Light and dark mode

• I have tested the changes in both light and dark mode
• The change is not relevant to dark and light mode

Accessibility

• Automated WCAG 2.1 tests pass
• HTML validation passes
• Manual WCAG 2.1 tests completed
• I have tested in a screen reader
• I have tested in high-contrast mode
• Any animations removed for prefers-reduced-motion
• Not required

Sustainability

• Images are optimised and lazy-loading used where appropriate
• SVGs have been optimised
• Performance and transfer of data considered
• If JavaScript is needed alternatives have been considered
• Not required

Pattern library

• The pattern library component for this template displays correctly, and does not break parent templates
• The styleguide is updated if relevant
• Changes are not relevant the pattern library

[chris-lawton]

Thanks @helenb - tested locally and all working as expected : )