Cannot quote deleted post

Clicking "Quote" on a post that has since been deleted throws the following error in an application using forem:

    Error Message:
    NoMethodError: undefined method `text' for nil:NilClass

    Where:
    forem/posts#new
    [GEM_ROOT]/bundler/gems/forem-ef6a4679928c/app/controllers/forem/posts_controller.rb, line 17

I wasn't sure how to add a new flash notice... it's just English. (I'm not sure if adding a translation for it to the `en.yml` file but not the others would cause problems.)

This pull request is intended for the master branch as well.