notify users that update is downloading

An important improvement that was discussed in legacy/trac#25694 (moved) is to let users know when an update is in the process of being downloaded. Firefox does not show this information in an obvious way; users need to open about:preferences and look in the Updates section or open the about box. Tor Browser users are sometimes confused because they know an update is available but have no easy way to know if it is being downloaded, and downloading the MAR files can take a while over Tor.

We plan to add a new "Downloading Tor Browser update..." message that will be displayed in the hamburger menu. We will also ensure that the standard "update" icon is displayed on the hamburger menu toolbar icon so users know to look inside for more info.