Upgrade to ETT 3.x.x

[elichad]

We're on version 2.4.0 currently. There are a few new features and bug fixes in 3.x.x already, including ones that affect the RO-Crate site.

Sub-tasks after an initial look locally:

• [ ] "Related pages" dropdown no longer works on domain pages
• [ ] Cards are highlighted in a dark colour on focus which affects contrast badly - tweak the colour or revert this to previous behaviour
• [ ] Links in cards should be colour $dark not blue (as the whole card is the link)