Add user_management and update app configuration pages

Currently, user_management page is on user_manual (written by @lateral-thinker).
This commit updates the page and moves it into admin_manual.

This commit also updates app configuration page with the new information.
Add what does sharing with user/group provides additionally

Sharing a file/folder gives extra options to choose from such as informing
user, giving creation, edit, reshare permissions. This commit adds
this information
Add what new button on web interface does

The new button has very nice features and it will be nice to provide
what it does and how it is used. Also navigating through the folders
is explained here.
Merge pull request #222 from wakeup/stable6

Add user_management and update app configuration pages