Add Admin model for Program Letter

[annagav]

Pre-Flight checklist

• [ ] Screenshots and design review for any changes that affect layout or styling
  • [x] Desktop screenshots
  • [ ] Mobile width screenshots
• [ ] Testing
  • [x] Code is tested
  • [ ] Changes have been manually tested

What are the relevant tickets?

Fix https://github.com/mitodl/micromasters/issues/5256

What's this PR do?

Add Admin model for Program Letter

How should this be manually tested?

Go to django admin and try to create a MicromastersProgramCommendation. Check the following things:

1. You can filter letters by program
2. you can see creation and update dates
3. you can create a MicromastersProgramCommendation successfully
4. you can search by user email

[codecov-commenter]

Codecov Report

Merging #5265 (dec7cac) into master (22fde05) will decrease coverage by 0.00%. The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #5265      +/-   ##
==========================================
- Coverage   93.68%   93.68%   -0.01%     
==========================================
  Files         501      501              
  Lines       23273    23286      +13     
  Branches      965      965              
==========================================
+ Hits        21804    21815      +11     
- Misses       1362     1364       +2     
  Partials      107      107              

Impacted Files	Coverage Δ
grades/admin.py	84.44% <84.61%> (+0.02%)	:arrow_up:

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