Admin users are not allowed for a project

In function isUserAllowed() it is only checked if:
1. any user is assigned to the project (if no then allow anybody)
2. if logged user is allowed for given project
But also _admin_ users _not assigned_ to the project shall be allowed.
Altered test cases for allowed user checking.

Changed test checking if admin used has always access.
Added new test to check if non admin used does not have access if is not added to acl list.
Merge branch 'patch-1' of github.com:toomyem/kanboard
Improve unit test
Add a 'due date' field and display the number of comments on the board
Assign existing tasks to nobody when a user is removed
Start to write some documentation about users management
Added Polish translations