refactor: move xmodule folder to root

- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
    - More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore

xmodule/css/html/edit.scss

@@ -0,0 +1,14 @@
+// HTML component editor:
+.html-editor {
+  @include clearfix();
+
+  .CodeMirror {
+    box-sizing: border-box;
+
+    height: 435px;
+  }
+
+  .is-inactive {
+    display: none;
+  }
+}