Reader comment likes

This PR implements likes for reader comments. Closes #1924.

@aerych May I bother you with a review?

I am a little unsure about how I handled the like/unlike requests, specifically not doing anything when it's success. My problem was that the `comment` object is not updated, so I couldn't pass the comment object to configure the cell. I assumed the cell will be reloaded when it's synced from the server again which would be ok. If this is a problem, please do let me know. If you have any suggestions, that'd be even better!

Also, in #1924 the screenshot is a little different than what we have, especially the color choice is grey in the actual app and blue in the screenshot. If we need to update the color for both the reply button and like button, I'd be happy to take care of that as well.

It was also kind of hard to test because of the bug #2512 which seems to be fixed in release branch but not yet merged into develop.
Same comment as before about a single method. You might look at `toggleLikedForPost:success:failure:` in `ReaderPostService`