hCAPTCHA updates from fable test

[thiessenp-cds]

Based on the result from the fable tests of the hCAPTCHA challenge (prompt). Note that these changes will not impact the current implementation that does not show a hCAPTCHA challenge. This is to prepare for further a11y testing.

The following could be improved

Can Fix:

• font colour (increase contrast) sort of
• background and border (increase contrast)

And the following would require an hCAPTCHA support ticket:

Cannot fix:

• font size (increase size to 16/18)
• challenge button text not matching button accessible name
• challenge missing a close button (but does close on ESC and clicking outside challenge)