Faulty URL handling in Markdown

URL handling in MarkdownText is not properly implemented. When we pass transformImageUri as urlTransform to ReactMarkdown, this applies for every URL found in the markdown text, not just images. The current way this problem is solved is by checking if the URL contains a slash and assuming that only in that case it is not an image. However, this breaks for some URLs, such as mailto:. This is especially bad because any time you enter an email address into a markdown capable field, it automatically infers a mailto: hyperlink, which as a result of this won't work.