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