Improve icons and add manifest to improve installing/mobile experience

[mrwweb]

With trends toward Progressive Web Apps, new support for installing websites in MacOS/iOS, etc. it strikes me as a good idea to add/update the following things to Plans:

• Higher-res default favicon
• New optimized icons for various OS's
• A site.webmanifest file that defines name (Grinnell Plans), short_name (Plans), start_url (https://grinnellplans.com/home.php), and probably the display type (standalone?). I don't think defining a theme_color or background_color makes sense as that seems likely to clash with most stylesheets and light/dark preferences

Over time, this would improve the experience for people "installing" plans on their home screens/task bars/etc.

I can't think of a downside, but I am very curious if people can think of extra considerations or potential downsides. I'm happy to submit the PR if at least a couple people think it's worth doing.

[iangreenleaf]

This all sounds like good stuff, and like it's unlikely to cause problems with anything else and therefore shouldn't require much regression testing. 👍 from me.