XYZ-37: Advanced Permissions Phase 1 Backend. (#8159)

* XYZ-13: Update Permission and Role structs to new design.

* XYZ-10: Role store.

* XYZ-9/XYZ-44: Roles API endpoints and WebSocket message.

* XYZ-8: Switch server permissions checks to store backed roles.

* XYZ-58: Proper validation of roles where required.

* XYZ-11/XYZ-55: Migration to store backed roles from policy config.

* XYZ-37: Update unit tests to work with database roles.

* XYZ-56: Remove the "guest" role.

* Changes to SetDefaultRolesFromConfig.

* Short-circuit the store if nothing has changed.

* Address first round of review comments.

* Address second round of review comments.

api4/channel.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 
 	l4g "github.com/alecthomas/log4go"
+
 	"github.com/mattermost/mattermost-server/model"
 )