use symbols instead of icons - #26

Here are my thoughts about this change:

• Switching from deprecated Google Icons to new Google Symbols is a great improvement!
• This is a major breaking change, as icon names and sometimes designs are changing.
• When we update this package in React UI, we'll also need to update the projects where we want to use the new React UI version.
• I've written some search+replace regexes to migrate to the new icons, but there are a few drawbacks:
  • Some icons might not exist in the new symbols or may have changed names. These will require manual fixes.
  • The old icons were sometimes outlined even when we used the normal (filled) version. The new icons have more filled and outlined versions, which means an icon might change from outlined to filled after migration. To be sure, we have to check each icon manually.