Fix accessibility errors

[dankohn]

https://tourmaline-salsa.glitch.me/
https://webhint.io/scanner/e5e3ec29-d41a-4bbb-92e0-189d3c70986b

Most of them can be fixed with alt tags and starker contrast.

[jordinl]

There are two more additional issues on the modal page: https://webhint.io/scanner/3a54bce0-1320-4bb3-b7ee-a4d85faddd42

The ARIA roles used must conform to valid values error happens because the modal (from Material UI) has role="none presentation". Webhint is giving an error because multiple roles is not well supported dequelabs/axe-core#1916. On the other hand multiple roles are actually allowed: https://www.w3.org/TR/2014/REC-wai-aria-implementation-20140320/#mapping_role

WAI-ARIA allows multiple roles to be specified as an ordered set of space-separated valid role tokens. The additional roles are fallback roles similar to the concept of specifying multiple fonts in case the first choice font type is not supported.

The ARIA hidden element must not contain focusable elements happens because Material UI adds aria-hidden=true to elements outside the modal, but one of the divs with aria-hidden=true contains an element with tabindex=0 which causes this error. There's an issue opened with axe-core discussing if this is a false positive: dequelabs/axe-core#1482

I think there's nothing we can do to fix these right now.

[jordinl]

Closing with #484