Add new enrollment message to the dashboard

The body of the enrollment message template

Tokenize platform name in message.

Changing to a datetime enrollment approach

Adding sorting. A little refactoring.

Adding confguration model for time delta

Adding admin registration and basic form for new config model.

Fixing docstring typo

Updating default time delta to 0, adding test to show it disabled functionality.

Removing the form from configuration and tweaking the enrollment message html
Merge pull request #5420 from edx/sanchez/enrollment-success-message

Add new enrollment message to the dashboard