add file manager to ispconfig

In short: Solve feature request [login to view URL] We want to offer file browser access to clients and admins. Things to note: - We have not decided on a particular solution - separate web user per site - restrict a site user to it's own files - access via ISPconfig panel login would be nice for admins and power users - Access per site would also be nice with separate credentials . - multi ispconfig server compatibel - security first, defensive programming - target is to get it merged into ISPconfig, follow the code style. - Documentation on how to set it up and how to maintain it.