Gif Block: Ability to set a size

#### Is your feature request related to a problem? Please describe.
Right now when you use the Gif Block, it inserts a gif that's as wide as your post/page's content area, and you don't have an option to scale it down.

#### Describe the solution you'd like
Add a control to set an image size, similar to the Image block, so the gifs can be scaled down.

#### Describe alternatives you've considered
Either adding a max-width on the theme level -- which may not always be how you want gifs to be sized -- or adding classes to the Additional CSS field and including custom styles.

Start by locating the Gif Block and Image block entry points and comparing how the Image block exposes size controls. Done means the Gif Block offers a comparable control and allows inserted GIFs to be scaled below the post or page content width.

Written by the indexing model from the issue text.