Fix missing metadata for system notes

What does this MR do and why?

Issue: #370852 (closed)

This fixes an issue where SystemNoteMetadata for a Note related to tasks or issue type changes have not been saved as the model was invalid.

This was visible for users on the activity stream of the issue as it didn't display an icon. We're now using the link icon for relating tasks to issues.

While adding a test for checking persistence, the issue_type also failed and got fixed. However, for issue_type we have not yet committed to an icon.

Note: This is not fixing the missing data from existing system notes.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Without this branch:
   1. Create an issue/use an existing one
   2. Create tasks in that issue
   3. Remove these tasks
   4. See the activity log of the issue (icons are missing)
2. With this branch:
   1. Create an issue/use an existing one
   2. Create tasks in that issue
   3. Remove these tasks
   4. See the activity log of the issue (icons are present)

MR acceptance checklist

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