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/
Firefox Aurora notes uses a different CSS scheme that needs an inversed logo, like here: https://www.mozilla.org/en-US/firefox/34.0a2/auroranotes/. So unless Thunderbird requires this also, I think we're good to remove the extra logo.