Add GA session_id to stub attribution data (Fixes #12355)

[alexgibson]

One-line summary

• Adds a new session_id parameter to stub attribution that contains the GA session ID.
• Renames visit_id to client_id as to be more technically correct / less confusing.

Significant changes and points to review

This change has already been tested end-to-end by QA against demo3, so code-level review should be all that's really required here. If you would like to manually test things still, there are testing steps below for the bedrock side of things.

Issue / Bugzilla link

#12355

Testing

1. On a Windows machine, open https://www-demo3.allizom.org/en-US/
2. Inspect cookies using dev tools and look for a cookie called moz-stub-attribution-code. The value should be a base64 string.
3. Decode the cookie value and verify it has both a client_id and session_id parameter.

[willdurand]

Hey, so this PR has been verified by QA and we update the stub service on production. Alex, you can go ahead with this PR now, thank you!

[stephaniehobson]

A few lingering references to visit ID but nothing blocking.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.12 🎉

Comparison is base (e868d1e) 76.75% compared to head (7f5b617) 76.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12794      +/-   ##
==========================================
+ Coverage   76.75%   76.88%   +0.12%     
==========================================
  Files         144      144              
  Lines        7674     7669       -5     
==========================================
+ Hits         5890     5896       +6     
+ Misses       1784     1773      -11     

Impacted Files	Coverage Δ
bedrock/firefox/views.py	91.55% <ø> (ø)
bedrock/mozorg/templatetags/misc.py	100.00% <0.00%> (+2.66%)	⬆️

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