improved nginx http to https redirect

"if" constructions in nginx should be used only if no other solution is possible, because "if" is the slowest directive and adds performance penalties ("If is Evil"). This merge request is supposed to improve this issue and adds another "server" block instead of using "if" directive. This is a "standard" approach which is widely used in nginx configurations for http->https redirections. All other functionality and features of ISPConfig are preserved.

Closes #2209 (closed) (redirection without "if" directive) Closes #3118 (closed) (the "split" is achieved using "server" blocks without "if" directive)