Replace content warnings in the renderer with linter

[mrdrogdrog]

Which part of the project should be enhanced?
Display of markdown issues.

Is your enhancement request related to a problem? Please describe.
We have warnings in the rendering that indicate problems with the content or the original source code. The warnings are useful but imho they shouldn't be visible in the renderer.

Describe the solution you'd like
Use the codemirror gutter (that the part where e.g. the line numbers are displayed) to display the warnings there. This way the warning is much closer to the source code that is causing that issue.

Additional context