Implemented system messages
Fixed exception "A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe." with base.BeginExecute in BaseController
Small UI improvement in the submissions list
Moved CompileExecuteAndCheck code to the ExecutionStrategy base class in order to be reused by other execution strategies
Admin links replaced with icons in contests list
Synced the latest code from the TFS: changeset 1797 build 2729