[Finishes #69042946] Browser support warnings

There are two parts to this PR: 0c23d18 adds warnings for old Firefox and Safari as requested.  The remaining changes add a new gem to abstract away browser detection.  After talking to Geoffrey I opted for gem 'browser' over 'user_agent_parser' due to better mobile detection support in Ruby.

Note that `browser.mobile?` only refers to phones (previously it meant both phones and tablets).