add helper function for checking if user is ignored

[tessalt]

What does this PR do?

We were getting errors Cannot read properties of null (reading 'some') from here. Obviously we need to add a check for user.ignoredUsers before checking .some on that array, but when I looked for this pattern elsewhere, I noticed multiple // TODO: extract function notes, so I extracted it into a function.

These changes will impact:

• commenters
• moderators
• admins
• developers

What changes to the GraphQL/Database Schema does this PR introduce?

none

Does this PR introduce any new environment variables or feature flags?

NA

If any indexes were added, were they added to INDEXES.md?

NA

How do I test this PR?

• with notifications enabled, as user 1, reply to user 2
• as user 2, ensure you have received a notification
• as user 1, ignore user 2
• as user 2, reply to user 1
• as user 1, ensure that you DO NOT receive a notification
• as user 1, un-ignore user 2
• as user 2, reply to user 1
• as user 1, ensure you DO receive a notification

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[kabeaty]

looks good to me, and all manual testing functions as expected!

[nick-funk]

good fix, thank you!