Created commit_on_success_with_read_committed decorator.

This decorator executes the decorated function inside a
transaction with isolation level set to READ COMMITTED.

TNL-367
Tests for the commit_on_success_with_read_committed decorator.

TNL-367
commit_on_success_with_read_committed should only work with atmost
1 level of transactions.

TNL-367
Added commit_on_success_with_read_committed decorator to views
which raise IntegrityErrors.

TNL-367
Merge pull request #5366 from edx/usman/tnl367-transaction-level-decorator

MySQL transaction isolation level decorator.