Revert "omd_backend.ml: changing id='something' to id="something""

This reverts commit d902e7008703fe28437bbb90955611d8d3c83107.
This reverts commit c136b054d4f33362b9da357aa9c4a35ad1f98a6e.

- StringSet moved to omd_backend.ml.
- fix: double line breaks in list items followed by a h1..h6