Correctly pluralize participants on issue show page

Right now, if there is only one participants, the issue show page says "1 participants" instead of "1 participant."  This commit uses pluralize() to fix that.
it will produce doubling number of participants

```
pluralize(1, 'person')
# => 1 person
```

so 

```
#{@issue.participants.count} #{pluralize(@issue.participants.count, 'participant')}

3 3 participants
```