Migrate functions to app package (#5106)

* Refactor and move session logic into app package

* Refactor email functions into the app package

* Refactor password update into app package

* Migrate user functions to app package

* Move team functions into app package

* Migrate channel functions into app package

* Pass SiteURL through to app functions

* Update based on feedback

cmd/platform/channel.go

@@ -5,7 +5,6 @@ package main
 import (
 	"errors"
 
-	"github.com/mattermost/platform/api"
 	"github.com/mattermost/platform/app"
 	"github.com/mattermost/platform/model"
 	"github.com/mattermost/platform/utils"
@@ -166,7 +165,7 @@ func removeUserFromChannel(channel *model.Channel, user *model.User, userArg str
 		CommandPrintErrorln("Can't find user '" + userArg + "'")
 		return
 	}
-	if err := api.RemoveUserFromChannel(user.Id, "", channel); err != nil {
+	if err := app.RemoveUserFromChannel(user.Id, "", channel); err != nil {
 		CommandPrintErrorln("Unable to remove '" + userArg + "' from " + channel.Name + ". Error: " + err.Error())
 	}
 }