Add server-side-rendering option "search engines only"

[markusjwetzel]

Server-side-rendering is most useful for search engines, so that they can follow links according to the source code. For the user experience it depends. On one hand the whole page could be loaded faster with server-side-rendering, but on the other hand the user has to wait longer to see the first piece of the page.

This said it might be useful to have an option that detects search engines and only enables server-side-rendering for these.