Frontsite a11y improvements

[JakobMiesner]

closes: CERNDocumentServer/cds-ils#863

Note: This PR was only tested with running it through cds-ils. The cookiecutter site was not tested.

This PR mainly focuses on making the frontsite more accessible. Some backoffice components/pages also got more accessible by proxy.
More specifically, a focus was put on the pages: /, /profile, /search and /literature/{pid}.
Changes include:

• making Popups more accessible (unfortunately limited by Semantic UI, see below)
• adding aria tags to buttons/inputs
• improving contrast
• fix heading hierarchies
• adding alt tags for images

PopupIcon

PopupIcon uses aria-label instead of aria-labelledby/aria-describedby:

Semantic UI React imposes limits here:

• Icon renders an <i>. so role="button" and tabIndex are bolted on manually.
• Popup renders its content in a portal that only exists in the DOM while open (I think?), and exposes no API to associate the popup with its trigger (no id on the content element or aria-describedby wiring). Referencing visible sibling text via aria-labelledby would be cleaner but requires changes in Semantic UI.

As a fallback the popup text itself is used as the icon's aria-label, so screen reader users get the full content on focus.