Issue completed tasks on stats

This PR changes the total answers per day graph under a project statistics for another that shows the number of tasks completed each day. This solves an issue with projects that reduced their redundancy for tasks and had many "extra" answers that were being shown in the graph, resulting in not very coherent graphs.

Before merging, merge the submodule themes changes too: PyBossa/crowdcrafting-theme#37 and PyBossa/pybossa-default-theme#46