Fix accessibility issues

[alistair-marshall]

Running lighthouse on Chrome reveals a few accessibility issues that need to be fixed:

• Contrast
• Form labels

After performing another audit using Wave:

• Add Alt text to help question marks
• Add labels to form data spending floor/ceiling values
• Add labels to name of simulation (do we even need this form element?)
• Three empty table headers (replace empty with empty )
• Fix labels for modal popups.

[alistair-marshall]

Table form labels fixed with #40.

[alistair-marshall]

After fixing the form labels, the lighthouse score jumped to 98%.

The contrast issues are the blue headers and the green buttons. I am fairly sure these are default bootstrap choices and are still pretty clear.

The colours will probably be changed if the project gets rebranded, so this bug can wait for now.