Fix notification when group set to watch (issue #395)

What does this MR do?

Fixes notification for group watchers.

There are two problems:

• When user is not part of project's team, but part of the project's group, and set group notifications to Watch
• When global notifications set do Disabled, but group notifications level set to Watch

Are there points in the code the reviewer needs to double check?

There aren't specs covering all scenarios described in notifications.md.

Why was this MR needed?

See Issue #395 (closed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Issue #395 (closed)