Make local file:... links navigate to the relevant file

Make a link like [[file:foo/bar.org][bar]] into href="foo/bar.org"
without target="_blank", so that it opens in the same organice window.
This allows more convenient navigation around a network of .org files,
and in particular an index.org file which facilitates quick jumping to
the most commonly used other .org files.

src/components/OrgFile/components/AttributedString/index.js

@@ -20,12 +20,19 @@ export default ({ parts, subPartDataAndHandlers }) => {
           case 'link':
             const uri = part.getIn(['contents', 'uri']);
             const title = part.getIn(['contents', 'title']) || uri;
-
-            return (
-              <a key={part.get('id')} href={uri} target="_blank" rel="noopener noreferrer">
-                {title}
-              </a>
-            );
+            if (uri.startsWith("file:")) {
+              return (
+                <a key={part.get('id')} href={uri.substr(5)} rel="noopener noreferrer">
+                  {title}
+                </a>
+              );
+            } else {
+              return (
+                <a key={part.get('id')} href={uri} target="_blank" rel="noopener noreferrer">
+                  {title}
+                </a>
+              );
+            }
           case 'percentage-cookie':
             className = classNames('attributed-string__cookie-part', {
               'attributed-string__cookie-part--complete':