Allow Decimal Points for Assignments

closes EGG-515
flag=none

This allows users to put decimal values in the
CreateEditAssignmentModal on the assignment index page.
It also provides error messaging for invalid and negative
point values

Test Plan:
- Go to Assignment Index page on a course
- Create a new assignment using the "+" button
  on the top of an assignment group
- Fill out name and due date
- Input "-10" for points
 - verify that the error message dissapears
 - try to save
 - verify that another error message appears talking
   about negative numbers
- Input one million points "1000000000"
 - verify that you cannot save
 - an appropriate error message is rendered upon save
- Input a "0" and verify that you can save
- Input a positive decimal value
- verify that the assignment is created without error
- select the kebab menu for the new assignment and edit it
 - verify that the above inputs result in the same outcome
   with the modal in edit mode

Change-Id: Iaf8b7fc5616004294fa8a2d8ed3ed96ee599e7d5
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/370131


Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jason Gillett <jason.gillett@instructure.com>
QA-Review: Jason Gillett <jason.gillett@instructure.com>
Product-Review: Sam Garza <sam.garza@instructure.com>