Notifications Redesign

### Goal
The purpose of this Pull Request is to implement support for the redesigned Notifications DataModel.


### Details
In the new DataModel, html strings have been entirely nuked, and a simple markup mechanism has been implemented. 

Each Notification consist of a collection of *blocks*: each one of them is mapped to a visual representation, and rendered onscreen. This is handled by a single class, `NotificationDetailsViewController`, meant for general purpose rendering.

Corner cases are: automattcher notifications, and comment-y notifications, which are being handled in a different way. Either the Reader is pushed, or the legacy CommentsViewController class is used.


### Highlights
- A New CoreData Model has been added, nuking the legacy `Note` entity, and adding the brand new `Notification` entity. This has been done to prevent migration issues.

- New Notification Model objects have been added. Parsing logic can be found there.

- Legacy UI class'ess have been nuked: `NotificationsFollowDetailViewController`, `NotificationsBigBadgeViewController` and `NotificationCommentDetailsViewController`, along with several UITableViewCell subclass'ess.

- The entire Notifications flows have been migrated to Storyboards, with Autolayout. You can find (everything) in `Notifications.storyboards`.


### Associated Issues
Fixes #1249 and #1874