Blockly bundle locales

This PR dramatically reduces the amount of time to build our Blockly-based JavaScript apps.

Before (only building English and pseudolocalized locales):
```
$ time grunt rebuild
[...]
Done, without errors.

real	0m37.081s
```
After (building ALL locales):
```
$ time grunt rebuild
[...]
Done, without errors.

real	0m15.949s
```
It also reduces the payload size of the final files:
Before:
```
196218 /en_us/bounce.min.js
160441 /en_us/calc.min.js
227974 /en_us/eval.min.js
193261 /en_us/flappy.min.js
154157 /en_us/jigsaw.min.js
278952 /en_us/maze.min.js
341921 /en_us/studio.min.js
274640 /en_us/turtle.min.js
167866 /en_us/webapp.min.js
```
After:
```
 48375 /bounce.min.js
 12458 /calc.min.js
203850 /common.min.js
 18625 /eval.min.js
 45315 /flappy.min.js
 13356 /jigsaw.min.js
132228 /maze.min.js
117503 /studio.min.js
128540 /turtle.min.js
 25735 /webapp.min.js
```

The only tradeoff is that we now load four `script` tags in our `_blockly.html.haml` view partial, instead of one, to grab the proper localized strings from for the app-specific and the common bundles:
```haml
  %script{src: "#{blockly_path}js/common#{pretty ? '' : '.min'}.js"}
  %script{src: "#{blockly_path}js/#{js_locale}/common_locale.js"}
  %script{src: "#{blockly_path}js/#{js_locale}/#{app}_locale.js"}
  %script{src: "#{blockly_path}js/#{app}#{pretty ? '' : '.min'}.js"}
```

(edit: Separating out the dev-mode scripts stuff into a separate pull request.)
Changing a setting (which blockly I'm using!) is not something that should happen when doing a *build*. For example, I build blockly often to test that the build system works, but I **never** want to use that version. This change will silently break everybody who isn't a frequent blockly builder.

The right place for this is in a `rake install:blockly` task in the top-level Rakefile. This task should toggle the symlink using a setting such as `CDO.use_my_blockly`.

The goal is for everyone to get their particular build system preferences expressed in their `locals.yml` file such that they can later clone the repo, copy their `locals.yml`, `rake install`, `rake build`, and enjoy.