Make TinyMCE editor translatable

Combine TinyMCE's built-in translation scheme with the django's translation framework, so edX's translators can make translations on TinyMCE in edX's own translation platform. A pesudo-language-code ```edx``` is used to simplify the entire process steps.

I do this for the following reasons:
1. The set of TinyMCE's offical translation languages are not as the same that in edX;
2. TinyMCE use a separate hard-coded .js file for each language rather than using django's translation framework, and loading of TinyMCE will fail if the corresponding translation .js file does not exist;
3. Because of the above two reasons, we may need to introduce many files into TinyMCE's ```langs``` directory in order to make sure TinyMCE works fine in every language, and furthermore, if a new language translation is added, a new corresponding translation .js file should also be added.

There's no need for the backslashes in this string, and will likely confuse translators.