#559 Add markdown live preview tester in code browser pages

[sdaau]

When we're looking in the code browser (say, https://sourceforge.net/p/pdfcmykerlite/code/ci/master/tree/), README.md files are automatically displayed, which is great.

However, sometimes I need to make small changes to README.md and commit and push, before I can see the result. For instance, it took me some commits to figure out that markdown image link to a subdirectory is not ![alt:screenshot01](img/myimg.png "Screenshot 01") - but instead it is ![alt:screenshot01](./img/myimg.png?format=raw "Screenshot 01").

Instead of polluting my repo with tons of few-byte changes (and the waste of time to commit and then refresh the page) just to get the markdown to display as I want it on Sourceforge, I would like that, once I'm logged in to the site and look at my own repos, there to be a link "Markdown preview tester" or similar; upon clicking, it would open a tester either in a new page, or on the same page, consisting of a multiline text field, and a preview div (same as on StackOverflow). There I could paste markdown code, and have it rendered as if it is a markdown file at that level of the repository (i.e. the preview at the root of the repository with an img subfolder would render an image for ![alt:screenshot01](./img/myimg.png?format=raw "Screenshot 01"); but the preview in a test subfolder would not render the same stanza, since img is not a subdirectory, but a sibling, of test).