Improve manageContent tags on gogreen demo

In HGGE-392 we updated the existing cmseditlinks to manageContent tags, but this was done in a very limited way. This issue is about implementing the tags properly.

• there is still one template with an hst:cmseditlink tag. Stores the value in a variable, which cannot be done anymore.
• started with adding a cms-request.css to position the add content button on the news overview at the bottom of the list. But in other stylesheets there are specific styling instructions for .hst-cmseditlink that clash with the added styling, so that does not yet work.
• functionality of updating / adding new content should be added to more places.