Refactoring of the Access Table

This commit does a lot of the work of refactoring the access table in a table with id's instead of strings.

The result does compile, but has not been tested. It may eat your kittens.

models/migrations/migrations.go

@@ -21,6 +21,7 @@ type Version struct {
 // If you want to "retire" a migration, replace it with "expiredMigration"
 var migrations = []migration{
 	accessToCollaboration,
+	accessRefactor,
 }
 
 // Migrate database to current version
@@ -158,3 +159,8 @@ func accessToCollaboration(x *xorm.Engine) error {
 	}
 	return nil
 }
+
+func accessRefactor(x *xorm.Engine) error {
+	//TODO
+	return nil
+}