fix: fix various issues and bugs with dpp::emoji

[Mishura4]

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This PR fixes various bugs involving dpp::emoji, including a double free after copying an emoji with image data, as well as dpp::base64_encode reading past the end of the data sent to it. It also modernizes (a little bit) dpp::emoji, and adds utility functions to get a mime type and file extension from a dpp::image_type or a dpp::sticker_format.

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