Clear newsletter cache on delete

When we delete newsletters, we need to clear the newsletter cache.
Use the post_delete signal.

Bug 841426
# Branch: (no branch)
Merge pull request #25 from dpoirier/clear_cache_on_delete

Clear newsletter cache on delete
Send different welcome messages for newsletters

- if the API specified a particular welcome, send that (not used yet)
- else, if they subscribe to more than one newsletter, just send the
  default welcome
- else, send the newsletter's own custom welcome message

Bug 829278, 841426
# Branch: (no branch)
Bug 841426: represent English as 'en' not 'en-US'
Merge pull request #26 from dpoirier/use_newsletter_welcome

Send different welcome messages for newsletters
Bug 854957: Add newrelic.
Merge pull request #29 from pmclanahan/bug-854957-add-newrelic

Bug 854957: Add newrelic.
Bug 854957: Ignore newrelic if import or env var fail.
Merge pull request #30 from pmclanahan/bug-854957-add-newrelic

Bug 854957: Ignore newrelic if import or env var fail.
Merge pull request #28 from dpoirier/english-as-en

Bug 841426: represent English as 'en' not 'en-US'
Use default cache

Instead of requiring configuration of a separate cache for the
newsletter data, use the default cache.

Bug 841426
Merge pull request #27 from dpoirier/use_default_cache

Use default cache
Bug 855415: Add db index to token column.

* Reduce token column length to 40 (UUIDs are 36 chars)
* Install South.
Merge pull request #31 from pmclanahan/bug-855415-add-token-db-index

Bug 855415: Add db index to token column.
Bug 855472: Add pl locale to double optin.
Install sentry (raven) for exception handling.
[bug 859564] Add Student Ambassadors newsletter and custom views.
Merge pull request #33 from glogiotatidis/master

Add Student Ambassadors newsletter and custom views.