* 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
* Implement POST /posts endpoint for APIv4
* Implement GET /channels/{channel_id}/posts endpoint for APIv4
* Implement GET /posts/{post_id} endpoint for APIv4
* Implement GET /posts/{post_id}/thread endpoint for APIv4
* Skip team get if it's a DM channel in handlePostEvents
* 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
