update plan service to work with self-hosted and DEC

[nora-codecov]

Purpose/Motivation

In CodeCov, every Org has a plan, but if the customer is self-hosted or DEC, they also have a license.

When an Org has a license, the plan_user_count on their Org is not accurate - the seat count is held by the number_allowed_users field from the license.
Some self-hosted or DEC customers are getting a false positive on an alert that checks the seat count before allowing a user to view a repository, because the alert is using the plan_user_count field instead of the number_allowed_users field from their license.

Links to relevant tickets

https://github.com/codecov/internal-issues/issues/679

What does this PR do?

When checking plan_user_count in PlanService, check for a license, and if they have a license, return number_allowed_users as the value for plan_user_count. With this change, regular customers as well as self-hosted and DEC will get an accurate number back from PlanService - it will always return the paid seat count.

[codecov-notifications]

Codecov Report

All modified and coverable lines are covered by tests ✅

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.05%. Comparing base (d515dc3) to head (cf27007).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##               main       #747   +/-   ##
===========================================
  Coverage   96.05000   96.05000           
===========================================
  Files           814        814           
  Lines         18489      18493    +4     
===========================================
+ Hits          17760      17764    +4     
  Misses          729        729           

Flag	Coverage Δ
unit	91.78% <100.00%> (+<0.01%)	⬆️
unit-latest-uploader	91.78% <100.00%> (+<0.01%)	⬆️

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