fix: Validate program letters and serve active letters only

[arslanashraf7]

Pre-Flight checklist

• [x] Screenshots and design review for any changes that affect layout or styling
  • [x] Desktop screenshots
  • [ ] Mobile width screenshots
• [x] Migrations
  • [x] Migration is backwards-compatible with current production code
• [x] Testing
  • [x] Code is tested
  • [x] Changes have been manually tested
• [ ] Settings
  • [ ] New settings are documented and present in app.json
  • [ ] New settings have reasonable development defaults, if applicable

What are the relevant tickets?

5250

What's this PR do?

• Adds a new field is_active in MicromastersProgramCommendation.
• Validates the existing Program letters based on program completion and marks them as active or inactive.
• Updates the logic to serve the program letters on basis of is_active through the URL.
• Updates the logic on the dashboard to show View Program Letter link only if Program letter is active.

How should this be manually tested?

• Before moving to this PR make sure that you have some program letters in your local setup, Create a couple for which the user has completed the program, and also create some letters for which the program isn't complete
• Now, Move to this PR, And see that the migration is successful and the Program letters that don't have completed the program. are marked as is_active=False (Verify this behavior)
• Now open the dashboard for this Program and confirm that you only see View Program Letter link for active letters(MicromastersProgramCommendation)
• Now open a program letter for an active letter it should be opened in a link something like http://mm.odl.local:8079/letter/program/0b91f501-0583-4171-ac8e-7b82a4cf3c59
• Now mark the above letter as inactive, Make sure that there is no link(View Program Letter) on the dashboard, and also opening the letter directly from URL(e.g. http://mm.odl.local:8079/letter/program/0b91f501-0583-4171-ac8e-7b82a4cf3c59) throws 404.

Where should the reviewer start?

(Optional)

Any background context you want to provide?

(Optional)

Screenshots (if appropriate)

When a program letter is active(Ignore the completion of courses, i created this manually):

When a program letter is Inactive:

[pdpinch]

I wish we had added an admin view. I opened an issue: https://github.com/mitodl/micromasters/issues/5256