Bug 1080634 - Update the Install link to point to a more relevant help p...

...age + Provide different URLs for Desktop & Mobile
The main issue is with ESR, which [inherits from this template](https://github.com/mozilla/bedrock/blob/master/bedrock/firefox/templates/firefox/releases/esr-notes.html#L5) and would point to the mobile download and install support link if we merged this as-is.