Lots of changes for new working member policies

- Scheduling screen has been updated to support proxy member workers
- Members are now awarded a set amount of discount days
  per shift worked. As alluded to earlier, members may also now work
  shifts for other members on their account.
- The "Task" model has now been updated with two new fields:
  account_member and proxy_for. The member field is no deprecated.
- A new report has been created "Recent Unexcused Shifts" which
  presents a list of all unexcused shifts for each member in their
  recent history.