Refactor convertFromHTMLtoContentBlocks

Summary: One of the utility helper methods commonly used to manage state with Draft.js is the `convertFromHTMLtoContentBlocks` method, which converts an HTML string into a data structure used by Draft.js for tracking state. This is a helpful utility, but it was written in an overly complex way. This commits adds a new simpler implementation of `convertFromHTMLToContentBlocks`.

Reviewed By: flarnie

Differential Revision: D7158000

fbshipit-source-id: 3424cdeb03663dc6c5a194da3d317c6ab97507dc

Author: Nicolas Champsavoir

src/model/encoding/__tests__/__snapshots__/convertFromHTMLToContentBlocks-test.js.snap

@@ -660,6 +660,120 @@ Array [
 ]
 `;
 
+exports[`Should preserve spacing around inline tags 1`] = `
+Array [
+  Object {
+    "characterList": Array [
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "ITALIC",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "BOLD",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "BOLD",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "BOLD",
+        ],
+      },
+      Object {
+        "entity": null,
+        "style": Array [
+          "BOLD",
+        ],
+      },
+    ],
+    "children": Array [],
+    "data": Object {},
+    "depth": 0,
+    "key": "key0",
+    "nextSibling": null,
+    "parent": undefined,
+    "prevSibling": null,
+    "text": "Some stylised text",
+    "type": "unstyled",
+  },
+]
+`;
+
 exports[`converts deeply nested html blocks when experimentalTreeDataSupport is enabled 1`] = `
 Array [
   Object {