[feature] Add config option to expose custom emojis without auth (#4233)

# Description

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>
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Does as it says on the tin! Should make things a bit easier for clients that don't provide an access token to the custom emojis endpoint.

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/2430

## Checklist

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- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
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- [x] I/we have performed a self-review of added code.
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- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
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Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4233
Reviewed-by: Daenney <daenney@noreply.codeberg.org>
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/api/util/scopes.go

@@ -31,6 +31,7 @@ const (
 	scopeBlocks        = "blocks"
 	scopeBookmarks     = "bookmarks"
 	scopeConversations = "conversations"
+	scopeCustomEmojis  = "custom_emojis"
 	scopeDomainAllows  = "domain_allows"
 	scopeDomainBlocks  = "domain_blocks"
 	scopeFavourites    = "favourites"
@@ -65,6 +66,7 @@ const (
 	ScopeReadBookmarks          Scope = ScopeRead + ":" + scopeBookmarks
 	ScopeWriteBookmarks         Scope = ScopeWrite + ":" + scopeBookmarks
 	ScopeWriteConversations     Scope = ScopeWrite + ":" + scopeConversations
+	ScopeReadCustomEmojis       Scope = ScopeRead + ":" + scopeCustomEmojis
 	ScopeReadFavourites         Scope = ScopeRead + ":" + scopeFavourites
 	ScopeWriteFavourites        Scope = ScopeWrite + ":" + scopeFavourites
 	ScopeReadFilters            Scope = ScopeRead + ":" + scopeFilters