Fix Quill editor to use div instead of p for blocks.

This fixes extra blank lines in the HTML output and relates to what
appears to be a bug/feature of all WYSIWYG editors I have evaluated,
including draft.js & slate.

kanmail/client/components/send/SendApp.jsx

@@ -16,6 +16,13 @@ import { closeWindow } from 'window.js';
 import { cleanHtml } from 'util/html.js'
 import { get, post } from 'util/requests.js'
 
+import Quill from 'quill';
+
+// Use divs > p, see: https://github.com/quilljs/quill/issues/861#issuecomment-239961806
+var Block = Quill.import('blots/block');
+Block.tagName = 'DIV';
+Quill.register(Block, true);
+
 
 function makeContactLabel(contactTuple) {
     if (!contactTuple[0]) {