Lea user can delete own posts

Added delete to posts. Converted "allowed_to_modify?" and "is_owner?" methods to be take any object as an argument.

@garthgoldwater @amZotti 
admins, moderators, and users will all be allowed to edit, but only owners and admins will be allowed to delete. I was trying to make this agnostic re: which user type is passed in, by extracting the owner type out to its own method. I'll do the same for admins and moderators, when we add them.

Ultimately, there will be an allowed_to_modify? and allowed_to_delete?, each of which will call the relevant user type methods for that action, at which point is_owner? will be a private method with the other two user type methods.

allowed_to_modify?(object)
----is_owner? || is_admin? || is_moderator
end 

allowed_to_delete?(object)
----is_owner?(object) || is_admin?(object)
end