Fixed comment form in projects#masterbranch

When trying to view an image, the following error was shown: 

`undefined local variable or method 'comment' for #<#<Class:0x00000003b82fb0>:0x00000003ab4c50>`

```ruby
= form_for(comment, remote: ajax) do |f|
  = f.hidden_field(:polycomment_type, :value => type)
  = f.hidden_field(:polycomment_id, :value => id)
  = f.hidden_field(:issue, :value => false)
```

Added instance variable to controller, and passed it into the comments/new partial.