refactor!: move RequestReactionType to model

[vilgotf]

Whilst working on the HTTP ratelimiter, I discovered this specialized ReactionType model. Like other specialized models, they should go into twilight_model::http. Additionally, the custom emoji optional name field is removed as provides no utility.

I also temporarily depended on this change, but I've since refactored that code.

[Erk-]

LGTM, I think the documentation update is good as well

[zeylahellyer]

I feel as though we should still have the name here, even if Discord doesn't end up using it. Who knows, there may come a day it does. The documentation for creating a reaction on a message is:

To use custom emoji, you must encode it in the format name:id with the emoji name and emoji id.

• https://discord.com/developers/docs/resources/channel#create-reaction

This reads to me like the API prefers the name. We should at least give users the ability to provide it so we match the API, regardless of whether it uses it.

[itohatweb]

Please resolve the merge conflicts and address zeylas review.

[vilgotf]

This might not be necessary and either way it's on the backburner for now.

[Gelbpunkt]

This might not be necessary and either way it's on the backburner for now.

Why would this not be necessary? Seems like a reasonable change to me.