Fix and add Accessible Names and improve tooltip accessibility

[markusjt]

Accessible Name

1.1.1: Non-text Content, 2.4.4: Link Purpose (In Context): Add alt text for CESSDA logo (header and footer).
1.3.1: Info and Relationships: Make sure labels are programmatically tied to input fields (search filters, results per page, sort by), check aria relationships + accessible name of language selector and add (non-visible) label for tooltips and search (both currently rely on icons and positioning for understanding their usage).
1.3.3: Sensory Characteristics: Add aria-label for < (previous page of search results) and > (next page of search results) pagination buttons. Should also help with understanding the other pagination buttons so for example 1 can probably just be 1 instead of "Page 1 of search results".

This issue is focused on accessible names even if some other minor accessibility issues might be fixed at the same time but for example landmarks are also part of 1.3.1 but will still be in a separate issue.

[markusjt]

Issues still remaining: Not possible to easily add accessible names to elements from searchkit: SearchBox (main search in header), InputFilter (keywords filter) and RangeFilter/RangeSliderInput (Collection years filter). They don't support adding aria-label (or aria-labelledby) or id.

Issue with empty aria attributes in language select is a bug in react-select that has been fixed in a later version: JedWatson/react-select#5131.