Feat: add properties for UTM codes

[lalver1]

Closes #2034.

Uses request.GET.get() to get the UTM code. If a UTM code is not present in the query string, returns null for that UTM code.

How to test

Using a local instance of benefits, send a request that looks something like http://localhost:port/?utm_campaign=transit. Verify that the property utm_campaign has been added to event_properties and user_properties.

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[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
benefits/core
analytics.py
Project Total

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[lalver1]

Looks like test_Event_update_event_properties() and test_Event_update_user_properties() in test_analytics.py already test the feature added in this PR, so new tests may not be required.

[thekaveman]

Looks like test_Event_update_event_properties() and test_Event_update_user_properties() in test_analytics.py already test the feature added in this PR, so new tests may not be required.

True, those functions have been tested. But this PR is adding behavior to the ViewedPageEvent, and so tests could (should) be written for that.

[lalver1]

Thanks, that makes sense. I added 4 new tests to test the new behavior of ViewedPageEvent. They are very similar to the Event tests but I think that this is ok since the new behavior of ViewedPageEvent is similar to the behavior of Event.

[thekaveman]

Nice work to keep on this @lalver1! There's still some refinement needed for the tests, hopefully my comments make sense. LMK if you want to chat.

[thekaveman]

Fantastic work @lalver1! Thanks for sticking with the tests, looks great!

[angela-tran]

This looks great!

[lalver1]

Thanks @thekaveman and @angela-tran!