Reactions list modal improvements

[dsevillamartin]

Changes proposed in this pull request:

• Allow deleting individual post reactions in post reactions list
• Fix emojis being too large in modal
• Fix showing if user has voted on the post already

Reviewers should focus on: This logic does not work for anonymous votes. It'd make sense to allow to delete an entire reaction type (I really hate the terminology being the same 😮‍💨), but I wasn't sure what API URL to use for that. As it stands already, it's not clear whether the endpoint is for a post_reaction or reaction ID... and since /posts/{id}/reactions returns post_reactions, I don't know whether the mismatch can be avoided at all.

Screenshot

Confirmed

• [X] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).
  • I haven't looked at tests at all. I know I could make them, but didn't feel like putting the work to learn & have the setup ready rn

[dsevillamartin]

Haven't implemented Pusher or tests. For Pusher, probably would want another event that just updates the count of the reaction type and have front-end handle if the user's reaction has been deleted... otherwise would require many events to be fired for deleting reaction types off posts.

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