Improvements to Image block

[thibaudcolas]

Extracting some of the tasks highlighted in #75 into a separate issue:

• Remove save / cancel buttons and save onChange. (Remove the buttons form the image dialoge WIP #102)
• Add a 'remove' button similar to the link tooltip.
• [x] Have image respond to align choices
• Add an 'edit' button similar to the link tooltip.
• Remove the dotted line around the image and center it. Make the image it's true size unless it is bigger and in that case make it's width 100%.

The following aren't possible in Wagtail at the moment (well, not in rich text), so removing those for now:

• [ ] Ability to add a caption on the image instead of alt text (which should be added on image upload (this maybe be specific to wagtaildraftail though).

[thibaudcolas]

@vincentaudebert I think this is something you could look at next.