Improvements to 'add task' page

I am going to have to change some things in master, though, because I have renamed the `creation_date` and `due_date` columns to `start_date` and `end_date`. But here's a summary of changes:
- Got better datepicker plugin, now uses it to enter dates instead of unwieldy textfields
- Two options for what to do after adding task: go to new task's page, or add another task immediately. No longer dumps you onto the homepage.
- Start dates can now be at any time rather than just the day the task was created. End dates are adjustable as well, although they are no longer called 'due dates' and they are now required.
- There is an option to send an email to someone who is assigned. It doesn't actually do anything because I can't test it locally, but hopefully we can figure it out on the server.