Moves NotificationItem in a separate file

[NejcZdovc]

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Auditors: @bsclifton

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

@NejcZdovc was this intended to be merged already?

[bsclifton]

Changes look good 😄 ++

[NejcZdovc]

Yes, this PR only splited two files, that's why I merged it without reviews. You can see changes in #9518.