Coding style commit.
Added assignment count data to project class (e.g., $this->needs[$need_id]['quantity_assigned']).
Factored out $this->shifts for the project class, replacing it with the clearer $this->open_needs. Added logic for distinguishing open needs from all needs.
Updated form logic to display only open needs.
Start phasing out the confusing "shift" language.
Updated tests for the definition of "open need," which is slightly different from the definition of "shift."
Merge pull request #200 from GinkgoFJG/limit_public_signup_to_open_shifts

Limit public signup to open shifts