Clarify unverified GPG Keys in Merge Requests
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### Problem to solve
When viewing changes in a Merge Request, a `verified` commit signature clearly states why a key has been verified, but lacks a justification for `unverified`
<details><summary>Screenshots</summary>
| Verified | Unverified |
| -------- | ---------- |
|||
</details>
### Proposal
### Scenarios
```rb
enum verification_status: {
unverified: 0,
verified: 1,
same_user_different_email: 2,
other_user: 3,
unverified_key: 4,
unknown_key: 5
}
```
issue