Investigate migrating to Plate

[cloverich]

After implementing and fixing a few issues with the current editor, it is quite complex for even basic features. The early releases I have in mind won't be doing anything sophisticated, so I want to look at using a more out of the box layer. Plate, which sits on top of Slate, appears to do that. Investigate migrating to it, and in particular closing out a few issues:

• remaining tasks in Support local images #36
• Better bullet lists #48
• migrate to plate
• Typing around links on their own lines jumps around #46
• validateDomNesting and trailing paragraphs: Ensure trailing p is present, but also denest images (by normalization, or in the mdast processing with visitor)