Add Opinion design tag and remove Opinion section

[alinaboghiu]

Max Walker has put together a proposal for introducing an opinion design tag instead of the existing section title:
https://docs.google.com/document/d/1Dui5QBfM2dqV19gL9qcFx42E4bJtVS1USd2V90AIFIM/edit#heading=h.7qwkxwa1tifr

[alinaboghiu]

There's a wip branch here (does not yet address removing the Opinion section title):
#9834

[JamieB-gu]

#10543 may make this easier to do.