[fix] Allow lowercase emoji shortcode in frontend

[moan0s]

Description

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This pull request fixes #1833

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[moan0s]

I did not add tests as I don't know enough about the project yet to do that :D I promise to learn that at some point

[tsmethurst]

There's a test failing here: internal/api/client/admin/emojiupdate_test.go:476 could you fix it?

expected: "{\"error\":\"Bad Request: shortcode  did not pass validation, must be between 2 and 30 characters, lowercase letters, numbers, and underscores only\"}"
actual  : "{\"error\":\"Bad Request: shortcode  did not pass validation, must be between 2 and 30 characters, letters, numbers, and underscores only\"}"

[tsmethurst]

Thanks!