Send unknown events to HipChat in red

If an event comes in to the test_app and has no matching template,
then send it to HipChat in red, not yellow, making it easier to
manage.
Update updateCard template to show diff

Using the trello_updates template tag.
Fix bug in sync_webhooks command

It was still referring to the old `Webhook.client` classmethod, instead
of using the new `get_trello_client` function.
Update the updateCard and updateList default templates

Better rendering of the updates (now as <li> items).
Update to missing template HipChat notification

Copy / format changes only.
Remove redundant welcome.html url and template

It was added because of the Heroku deploy button instructions, but
doesn't appear to be used? Also updated app.json.
Add CONTRIBUTING to the manifest for inclusion in installs
Set DEBUG based on environment variable of the same name
Improve syncing to flag callback_url mismatches

If the callback_url differs between the local and remote webhook there
is currently no way to update the remote (using py-trello); instead,
log a WARNING and set the trello_id to "INVALID" to make it obvious
that the local hook needs to be deleted and recreated.
Update settings to enable static serving from django

This is a test app, so we don't care about setting up a CDN - let django
deal without, good or not.
Disable the CallbackEventInline in admin

This works well in dev, but in the wild having every event render on
the same page for a single webhook is impractical.
Fix KeyError in trello_updates template tag

If there is a value in the 'old' node in the event payload, but no
matching value in the updated version, then return None.
Add partial template to render updates as a list

The trello_updates filter returns updates in a generic format, and the
update_values_list.html template renders these as a basic unordered
list.
Add tests for templatetags
Update app.json to have more useful information

I've added the 'description' and 'required' attributes to the various
environment variables.