This change is fixing "All Java build rules should be loaded from
Starlark" warning flagged by latest buildifier version: [1]. Python
rules are now also loaded from the Starlark.
Also extract codemirror library import to BUILD file. This is needed to
avoid cycle in the workspace file, after importing java rules from
Starlark.
[1] https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#native-java
Change-Id: I36192c9465d988b25cf09c250e110f15850910cd