Add a source on elements

Codecov Report

Merging #683 into master will increase coverage by 0.01%. The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master     #683      +/-   ##
==========================================
+ Coverage   86.37%   86.39%   +0.01%     
==========================================
  Files         187      188       +1     
  Lines       13654    13674      +20     
==========================================
+ Hits        11794    11813      +19     
- Misses       1860     1861       +1

Impacted Files	Coverage Δ
arkindex/documents/models.py	`32.32% <0.00%> (-0.11%)`
...kindex/documents/migrations/0005_element_source.py	`100.00% <100.00%> (ø)`
arkindex/documents/serializers/elements.py	`96.47% <100.00%> (+0.02%)`
arkindex/documents/tests/test_elements_api.py	`100.00% <100.00%> (ø)`

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