Janky Iceweasle Fork: What's Old Is New Tab

• Update to latest upstream Fenix
• Turn on download manager, login selector, and tab history features from upstream beta/nightly
• Improved README
• Dropped some totally broken add-ons from the list of available add-ons. See https://github.com/interfect/fenix/issues/7 and https://github.com/interfect/fenix/issues/33.
• Switched to a new, old-Fennec-style tab switcher UI courtesy of @abhijitvalluri. See https://github.com/interfect/fenix/pull/41. If you want your new-Android-style floating plus button back, demand a setting for it from @abhijitvalluri or someone else who knows how to do Android UI stuff.