Thunderbird RNA.

* Upgraded to the latest RNA
* Created releasenotes bedrock app
* Migrated firefox release notes views to releasenotes app.
* Added thunderbird releasenotes urls
* Added global.conf RewriteRules for Thunderbird release notes.
* Thunderbird base, release notes, system requirements templates

Note that you probably want to run:
 * git submodule update --recursive   # for rna update
 * ./manage.py update_product_details   # for /thunderbird/releases/
We can remove `/en-US` here