Add the __language__ symlink used by the khan academy build system.

Summary:
Add en.js as a symlink to en-ca.js.

I looked at en-ca.js manually, and it seems like it's appropriate for
the US as well, so this symlink should be safe.  If not, we'll have to
revisit what en.js looks like.

This new file is needed because inject_i18n_strings causes
__language__.js to point to en.js for unsupported languages (such as
'boxes'), so en.js must always exist in this directory.

Maybe the right fix is to change inject_i18n_strings.py to point
somewhere else if en.js doesn't exist, but this fix is easier.

Test Plan:
Ran
   make i18n_compile
and then saw the following:
% ls -l ~/khan/webapp/genfiles/translations/*/third_party/javascript-khansrc/moment-khansrc/min/lang/__language__.js
lrwx------  1 csilvers  staff  5 May 24 12:44 /Users/csilvers/khan/webapp/genfiles/translations/accents/third_party/javascript-khansrc/moment-khansrc/min/lang/__language__.js@ -> en.js
lrwx------  1 csilvers  staff  5 May 24 12:44 /Users/csilvers/khan/webapp/genfiles/translations/boxes/third_party/javascript-khansrc/moment-khansrc/min/lang/__language__.js@ -> en.js
lrwx------  1 csilvers  staff  5 May 24 12:44 /Users/csilvers/khan/webapp/genfiles/translations/es-ES/third_party/javascript-khansrc/moment-khansrc/min/lang/__language__.js@ -> es.js
lrwx------  1 csilvers  staff  8 May 24 12:44 /Users/csilvers/khan/webapp/genfiles/translations/pt-BR/third_party/javascript-khansrc/moment-khansrc/min/lang/__language__.js@ -> pt-BR.js

As expected, the es-ES and pt-PT symlinks point to es.js and pt-BR.js,
while the fake langauges point to en.js.

Fingers crossed.

Reviewers: joshnetterfield, john, chris

Reviewed By: john

Differential Revision: http://phabricator.khanacademy.org/D2433