Fix overriding cm-highlightSpace

With [0], it changed how to overide it. We fix this by using
cm-highlightSpace rather than cm-highlightSpace:before.

[0]
https://github.com/codemirror/view/commit/605fd723b9e18cc3523335f2f803418305b93b68

Change-Id: I7c2829e444ef7c0ac12910ff277e24ab5e052bad
diff --git a/web/element/extensions.ts b/web/element/extensions.ts
index 1f59c35..9aa8082 100644
--- a/web/element/extensions.ts
+++ b/web/element/extensions.ts
@@ -82,8 +82,9 @@
       content: "'\\2192'",
       position: 'absolute',
     },
-    '.cm-highlightSpace:before': {
-      content: "''",
+    ".cm-highlightSpace": {
+      'background-image': 'none',
+      'background-size': 'none'
     },
   });