Don't overwrite comment components if they're already set

Small fix. I'm finding that in a Rails 4.1 app (running rails-api) that the Marginalia instrumentation is being lazy loaded by ActiveSupport. This means it runs after I've set the comment components in my initializer, so they get overwritten. Switching the assignment to `||=` stops them being overwritten.

cc @arthurnn 
My thinking was that this will only be run once: at the point when the `Marginalia::Comment` module is required, and right after `components` gets defined.

That said, I'm totally happy to put the `||=` back in. It's the safe option.