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/
This menu item, as well as "Support" do not have any submenu items, so they still get the little carat on hover, and clicking them in the mobile nav menu throws a JS error. 

Looking at our existing mobile nav options, I don't believe mixing like this is currently supported. It's something we could possible fix going forward, but for now it might be worth finding an interim solution that works?