Fix accessibility issues in Convert-a-Card

[adikeinan]

Convert-a-Card would benefit from accessibility fixes. I ran this accessibility tool: https://web.dev/measure/ on Convert-a-Card (https://www.libcrowds.com/collection/convertacard), and there are some issues there. The most pressing for us to fix is the ones that fall under the 'Accessibility' category, in order to comply with UK web regulations.
This has an equivalent issue in 'In the Spotlight': #863

Next step: NIIT to estimate accessibility improvements and testing.

[adikeinan]

LibCrowds BL Accessibility Statement: https://www.bl.uk/about-us/governance/policies/accessibility-for-british-library-websites/libcrowds-accessibility-statement
Gov.uk Guidance: Understanding accessibility requirements for public sector bodies: https://www.gov.uk/guidance/accessibility-requirements-for-public-sector-websites-and-apps#things-you-might-not-need-to-fix