Added workflow to completely reset and clean the project.

Summary: This workflow (triggered by `arc force-reset`) causes the entire working directory to be reset and cleaned, and all submodules updated, reset and cleaned back to their expected state.  After running this command, there will be no unstaged or staged changes at all.

Test Plan: Change some files in your working directory and inside some submodules (`Build/cslint` seems to like being modified by MonoDevelop).  After running `arc force-reset` all changes in the working directory should be reset and all build artifacts deleted.  In `Git Extensions`, the "Commit" toolbar icon should display a green tick with 0 pending changes.

Reviewers: Sabre070

Reviewed By: Sabre070

Differential Revision: http://code.redpointsoftware.com.au/D51