fix(notifications-v2): Use notifications_service for project serializer

[snigdhas]

fixes SENTRY-1627
fixes SENTRY-1628
fixes SENTRY-1629
fixes SENTRY-162A

[scefali]

@snigdhas could you add a test for this? Probably just need to add a feature flag to an existing test.

[snigdhas]

@snigdhas could you add a test for this? Probably just need to add a feature flag to an existing test.

I'm ~85% sure this code runs but is never used anywhere - I can't find anywhere in the code where we use the value of is_subscribed. Historically, it doesn't look like this was ever included the serializer since Marcos worked on it a few years ago. I'll merge this and file a ticket to clean this up as part of the rest of the cleanup.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ TypeError: get_subscriptions_for_projects() got an unexpected keyword argument 'user' /api/0/organizations/{organization_slug}/projects/ View Issue
• ‼️ TypeError: get_subscriptions_for_projects() got an unexpected keyword argument 'user' /api/0/projects/ View Issue

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