Implement GET channels endpoints for APIv4 (#5363)

* 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

model/channel.go

@@ -15,6 +15,7 @@ const (
 	CHANNEL_DIRECT                 = "D"
 	DEFAULT_CHANNEL                = "town-square"
 	CHANNEL_DISPLAY_NAME_MAX_RUNES = 64
+	CHANNEL_NAME_MIN_LENGTH        = 3
 	CHANNEL_NAME_MAX_LENGTH        = 64
 	CHANNEL_HEADER_MAX_RUNES       = 1024
 	CHANNEL_PURPOSE_MAX_RUNES      = 250
@@ -83,10 +84,6 @@ func (o *Channel) IsValid() *AppError {
 		return NewLocAppError("Channel.IsValid", "model.channel.is_valid.display_name.app_error", nil, "id="+o.Id)
 	}
 
-	if len(o.Name) > CHANNEL_NAME_MAX_LENGTH {
-		return NewLocAppError("Channel.IsValid", "model.channel.is_valid.name.app_error", nil, "id="+o.Id)
-	}
-
 	if !IsValidChannelIdentifier(o.Name) {
 		return NewLocAppError("Channel.IsValid", "model.channel.is_valid.2_or_more.app_error", nil, "id="+o.Id)
 	}