Upgrade to Semantic UI 2.3

[nmoroze]

https://semantic-ui.com/introduction/new.html

[nmoroze]

Reopened due to a few breaking changes in Semantic 2.3 that I'm looking into.

Current work is on https://github.com/techx/cog/tree/update-semantic, was previously squashed & merged into master but has now been reverted.