splash page: add user groups section

If a user is signed in, they can see a "groups" section on the splash
page that lists all of their groups.

Currently the group tiles use an "empty" image with a background color.
The background colors rotate through the titleColors listed in theme.js.

static-site/src/components/splash/userGroups.jsx

@@ -26,13 +26,12 @@ const UserGroups = (props) => {
   return (
     <Fragment>
       <ScrollableAnchor id={'groups'}>
-        <Styles.H1>Private Nextstrain Groups</Styles.H1>
+        <Styles.H1> {props.user.username}&apos;s Groups</Styles.H1>
       </ScrollableAnchor>
 
       <FlexCenter>
         <Styles.CenteredFocusParagraph>
-          Nextstrain groups are collections of datasets with controlled access.
-          You ({props.user.username}) have access to the following groups:
+          You have access to the following groups:
         </Styles.CenteredFocusParagraph>
       </FlexCenter>