PR mode: show other potentially fixed issues by content and linked issues.

### Feature Request

unsight should show all issues that are not already linked for PRs.

For Ex 1: by looking at all the related issues of `#123` (linked via the description).

For Ex 2: by the context of the description and possibly the code.

This should enable us to potentially resolve other related issues that have been addressed incidentally.

### Ex 1:

#### PR:

```
fixes #123

This PR does stuff that should fix foo and bar
```

#### Issues:

unsight does already link `#123` and `#321`

##### `#123`
```
foo not working
```

##### `#321`
```
foo and bar not working
```

### Ex 2:

#### PR:

```
fixes #123

This PR does stuff that should fix foo and bar
```

#### Issues:

unsight does **NOT** link `#123` and `#321`

##### `#123`
```
foo not working
```

##### `#321`
```
bar not working
```