Courseware URL should be a computed property

[jkachel]

We ran into an issue where the courseware URL for a course had an error in it, which prevented people from accessing the course. Upon further reflection, though, a couple of things came up:

• The URLs all follow a specific pattern (that has actually recently changed, in a non-destructive fashion)
• We depend on the user to set this field correctly (import_courserun calculates this but I think it's the only thing that does)

Given that the URLs can be programmatically determined, and that MITx Online doesn't do third-party courses, it seems like this should be a computed property rather than a stored one.

Acceptance Criteria:

• The courseware URL is generated according to the following pattern (in pseudocode): <edX base URL>/learn/course/<readable id>/home
• The URL pattern is stored in a known/documented spot for future maintenance
• The field name does not change
• The database field is removed
• Existing management commands - specifically import_courserun and create_courseware but there may be others - are updated to ignore the courseware URL field
• Users are able to navigate to the course homepages successfully

Other

• Is there value in keeping the field around and then allowing for an override? My thought is no for this application but other apps (specifically, new open) may need an override if they use this pattern too.