The secure media functionality relied on `SiteSetting.enable_s3_uploads?` which, as we found in dev, did not take into account global S3 settings via `GlobalSetting.use_s3?`. We now use `SiteSetting.Upload.enable_s3_uploads` instead to be more consistent.
Also, we now validate `enable_s3_uploads` changes, because if `GlobalSetting.use_s3?` is true users should NOT be enabling S3 uploads manually.
Martin Brennan