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

# Description

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>
> If this is a documentation change, please briefly describe what you've changed and why.

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.
- [x] I/we have not leveraged AI to create the proposed changes.
- [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>

docs/swagger.go

@@ -41,6 +41,7 @@
 //   - read:applications: grants read access to user-managed applications
 //   - read:blocks: grants read access to blocks
 //   - read:bookmarks: grants read access to bookmarks
+//   - read:custom_emojis: grants read access to custom emojis
 //   - read:favourites: grants read access to accounts
 //   - read:filters: grants read access to filters
 //   - read:follows: grants read access to follows
@@ -99,6 +100,7 @@
 //	      read:applications: grants read access to user-managed applications
 //	      read:blocks: grants read access to blocks
 //	      read:bookmarks: grants read access to bookmarks
+//	      read:custom_emojis: grants read access to custom emojis
 //	      read:favourites: grants read access to accounts
 //	      read:filters: grants read access to filters
 //	      read:follows: grants read access to follows