Convert HEIC uploads to JPG by default (PR #10395)

This removes the convert_heif_to_jpeg site setting and adds .heic and .heif to the authorized extensions. The result is that uploads of HEIC images will automatically convert to JPG. (Sites that have modified their authorized extensions need to manually add .heic and .heif.)

Context: https://meta.discourse.org/t/support-for-heic-images/144326/20?u=pmusaraj

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