Comment hot fix

Closes #2929 
Refs #2894 
Adds back the ability to reply to and edit comments on a self-hosted site. 
Adds new functionality for displaying an avatar image when `avatarImageForDisplay` is nil 
Fixes an issue with how the return value of the call to wp.editComment was used. 
The right way to check if REST is available is `blog.restApi` as it's done in [remoteForBlog:](https://github.com/wordpress-mobile/WordPress-iOS/blob/92b7b6d5e20693b471b380f86db42312abd0b3cc/WordPress/Classes/Services/CommentService.m#L859).

However, I don't think the view controller should know or care about the specific network implementation. It should be able to tell `CommentService` "I want to update this `Comment` with this new content".

From what I understand, the point of the REST-specific methods was to act on comments when there was no `Comment` object (I guess notifications), so if you have `self.comment` you don't need `updateRESTCommentForNewContent:`