content [nfc]: s/image/image preview/, making room for new "inline image"#1923

chrisbobbe merged 2 commits intozulip:mainzulip/zulip-flutter:mainfrom

chrisbobbe:pr-rename-image-to-image-previewchrisbobbe/zulip-flutter:pr-rename-image-to-image-previewCopy head branch name to clipboard