Allow reacting to comments

Good idea, probably doable with flag module and some CSS.

As much as I love emoji, if we want to use this to identify unhelpful contributions as implied in #2972225: Move from opt in to opt out for contribution attribution, I wonder if an icon system conflates a thumbs-down for "This comment is not helpful" with "I disagree with this direction" (but even if a different direction is picked, discussing it is still valuable) or even "You've surfaced a gnarly problem!"

There's a very long issue somewhere promoting a simple upvote/downvote that things like Stackoverflow and Reddit have with lots of discussion and alternate proposals for more specific reactions; probably we should take all that into account before making this change? It got stalled because it's a difficult problem space for us.

If we want parity with GitLab, for some consistency between issues and merge requests, they actually allow every emoji https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25187. I’m not sure that this much flexibility is a good thing. On the issue/merge request summary, there are default 👍/👎 in addition to the emoji picker.

For #3, which is a great point, maybe we should wait until the greater crowd-sourcing community figures it out? #4 is probably a good place to start, as we currently have nothing.

#3 issues I'm worried about are (assuming we'd have a pick list of text reactions):

• How do we decide what reactions to provide?
• Who curates that list? (i.e. Is this one more thing for the Web team to deal with?)
• Can users arbitrarily add new reactions?

The Reaction and Voting API Reaction modules looks useful here, but they're only for Drupal 8. Maybe we can add this to the things-to-do-after-d.o-gets-upgraded-to-D8 list.

Is this issue still relevant with #3295357: Migrate drupal.org issues to GitLab issues well underway?

Not really.