feat: Added file size limit to avatars and banners

[Jaskowicz1]

This PR adds a file size limit to avatars and banners as Discord seemed to silently introduce an avatar file limit (either that or it's always been there and it wasn't properly working the other day).

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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