coqbot should be able to add reactions rather than just posting comments (e.g., for minimization)

[JasonGross]

For future reference: we want to edit https://github.com/coq/bot/blob/master/bot-components/GitHub_mutations.ml and https://github.com/coq/bot/blob/master/bot-components/GitHub_GraphQL.ml and the references are:

• https://docs.github.com/en/graphql/reference/mutations#addreaction
• https://docs.github.com/en/graphql/reference/input-objects#addreactioninput
• https://docs.github.com/en/graphql/reference/enums#reactioncontent