Added order reference numbers

[rhysyngsun]

Pre-Flight checklist

• [x] Migrations
  • [x] Migration is backwards-compatible with current production code
• [x] Testing
  • [x] Code is tested
  • [x] Changes have been manually tested

What are the relevant tickets?

Part of #5213

What's this PR do?

This adds the reference_number column and the code to populate that value on create going forward.

How should this be manually tested?

• Go to your dashboard and perform a checkout
• You only need to get as far as being redirected over to CyberSource (you don't need a functional Cybersource integration), once that happens you can go to django-admin and confirm your order has a reference number populated

[codecov-commenter]

Codecov Report

Merging #5214 (a2692a1) into master (92060cf) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5214   +/-   ##
=======================================
  Coverage   93.97%   93.98%           
=======================================
  Files         499      499           
  Lines       23155    23163    +8     
  Branches     1414     1415    +1     
=======================================
+ Hits        21761    21769    +8     
  Misses       1289     1289           
  Partials      105      105           

Impacted Files	Coverage Δ
ecommerce/models.py	95.63% <100.00%> (+0.17%)	:arrow_up:

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