Accessibility fixes -- "pa11y-ci"

[nfreear]

..

• https://github.com/pa11y/pa11y#command-line-interface
• https://github.com/pa11y/pa11y-ci#usage

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Travis-CI: IET-OU/openEssayist-slim @ job 341846042 :

• 8 Errors
• 6 Warnings
• 0 Notices (ignored)

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[iet:10305316]