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

# Description

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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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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>

testrig/config.go

@@ -92,6 +92,7 @@ func testDefaults() config.Configuration {
 		InstanceExposeBlocklistWeb:     true,
 		InstanceExposeAllowlist:        true,
 		InstanceExposeAllowlistWeb:     true,
+		InstanceExposeCustomEmojis:     true,
 		InstanceDeliverToSharedInboxes: true,
 		InstanceLanguages: language.Languages{
 			{