Add unsupported text for IE browsers (is currently blank)

[EddyIonescu]

Currently the web-app is blank when opening it in IE-11. While regular people don't use this, many employees at transit agencies (including Muni) do, despite having Chrome already installed.
If the browser is IE, we should display text saying something along the lines of "Internet Explorer browsers are not supported. Please use a modern browser such as Chrome, Firefox, Safari, or Edge to access OpenTransit".

[EddyIonescu]

Also, if you make a PR, I can test it out to ensure it works