Frontend/raven: install raven-js

How to test
----------------
1. It just work if `app.debug` is `False`. So you can test this PR with this guidelines.

   ```python
   # DEBUG = True

   # or

   DEBUG = False
   ```

2. Add Sentry's DSN into config file.

   ```python
   SENTRY_DSN = 'http://public_key:secret_key@example.com/1'
   SENTRY_INCLUDE_PATHS = ['cliche']
   ```
[Context processor example](http://flask.pocoo.org/docs/0.10/templating/#context-processors) was use inner function. `make_public_sentry_dsn` function really need globally?