Allow to add reactions for locked threads

[just-boris]

Sometimes maintainers of projects prefer to stop useless discussion and possible offtopic by locking the thread.

But then it is also not possible to add reactions to already existing comments. Actually, it breaks actual usage, because, after that, users are unable to vote for the existing issue and have to create a new one.

Here are some recent examples:
eslint/eslint#3458
babel/babel#2645

I'd like to vote for them, to show that this is important for me, but I can't as far adding reactions is not available for locked issues.

[just-boris]

Thank you @TPS, it makes some sense.
Updated the title and description

[tyteen4a03]

I think this issue also highlights the need for reactions in locked threads.

[Azarlak]

If this is to be implemented, then preferably both of these would be possible for users:
  A: Locking the comments only
  B: Locking both the comments and the reactions

I suppose one way of doing this would be to have the comments and reactions separately lockable/unlockable. This could also allow users to lock the reactions without locking the comments if they so desire; I don't know if that would ever be useful, but it is an interesting thought.

[vith]

The practice of doing a thumbs-up on the first comment in lieu of a public "star" or "vote" feature for issues breaks down when a legitimate, open issue thread has to be locked to collaborators only due to its popularity.

#37 for example, is an important open issue for github itself, but there's now no longer any way to signal a "me too" on it so that the org can measure how much impact the issue is having.

We either need the ability to continue adding reactions to the first comment at minimum, or a full "star" feature for issues with a public counter.