Add mobile releasenotes switcher to fix bug 965012

Add /mobile/{version}/releasenotes url to bedrock

Add equivalent_desktop_release and equivalent_android_release to
template context using new methods from rna Release model that
point to latest minor release

Add template logic using new template context variables to
conditionally add relnoteswitcher div with link to equivalent
version

Update rna submodule to latest version which has the new methods

Port #relnoteswitcher styles from firefox-notes.css to
releasenotes.less and adapt to new template