Changed schema of ThanksLog table

* Replaced IX_Objects index with a unique index composed by ObjectType, ObjectID and InsertUserID.
Refactored ThanksLogModel class

* Fixed bugs in ThanksLogModel::Delete() method.
* Added methods to delete a log entry by ThankID or by object key (object type, object ID, sender).
* Cleaned up obsolete code.
* Refactored code to adhere to internal standards.