Add mobile releasenotes switcher to fix bug 965012

    Add mobile releasenotes switcher to fix bug 965012
    
    Add /mobile/{version}/releasenotes url to bedrock
    
    Add releasenotes_url to bedrock.mozorg.helpers.misc
    
    Add equivalent_release_url helper function that uses new
    equivalent_desktop_release and equivalent_android_release methods
    from rna.models.Release that point to latest minor release and
    releasenotes_url helper method
    
    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
    
    Fix link to note.fixed_in_release using new releasenotes_url helper