* Update user.go
Added method to create user accounts using a privileged system_admin account
* Update user.go
Added respective method to handle the the APIv4 call to create users with privileged account(system_admin) by passing server restrictions of EnableUserCreation and/or EnableOpenServer set to false.
* Update user_test.go
Added test case
* Implement POST /users/search endpoint for APIv4
* PLT-2713 Added store functions for searching users that don't have a team
* PLT-2713 Added 'without_team' option when searching users
* PLT-2713 Added 'without_team' option when searching users (v4)
* added get session and revoke session endpoints, unittests and drivers
* removed BasicUser2 and added teardown
* added badrequest unit test case for sessions
* added session loop to check if user id and session user id matches
* fixed indentation issues for user_test
* match indentation from spaces to tabs
* implement get channels endpoints and updated drivers and unittests
* removed channel deletion on tear down, removed manage permission on get channels endpoints, and updated utils to add constant channel length
* added constants for user, team and channel length, updated context to use the model functions
* make sure team name length should be less than the minimum length and revert underscore to team name validity
* changed post test condition from notfound to unauthorized
* added get user by username endpoint
* added get user by username unit test and driver
* changed username length to 22 characters max
* changed Params to UserName to Username
* reorganized get user by username and get user by email formatting in model/client4
* added get user by email endpoint for APIv4
* added get user by email endpoint unit test and driver
* removed the appended return of user ids on logout
* Added RequireEmail to validate user email. Also updated the get user by email endpoint and unit test
