The Review::summary was limited in behaviour, since it was only an Option<String>.

The field is now changed to NonEmpty<Edit> so that it can enable an edit history, as well as being able to supply embeds as part of the text.

Reactions are also introduced to Review and, necessarily, introducing an Action::ReviewReact.

Note that some machinery was added under actions and encoding to enable backwards-compatibility. For actions, a new ReviewEdit is introduced to keep track of the different versions of editing actions – supporting the old actions while also introducing the new variant. For encoding, the machinery exploits the fact that an Option<String> can be converted into a NonEmpty<Edit> given that an ActorId and Timestamp are available, and that None can be converted into an empty string. The struct patch::Review now deserializes via this encoding::review::Review type and, importantly, preserves backwards-compatibility.

Rebase and clear up test.