Add `/group/:pubid/edit/members` route and placeholder page in frontend

[robertknight]

Add a route for editing group members and a placeholder page in the frontend. When the group_members feature flag is enabled, the group forms display a tab bar that allows the user to switch between the pages for editing group settings and group members.

Summary of changes:

• Add a /group/:pubid/edit/members backend route
• Expose group_members feature flag to frontend
• Add a router component (AppRoot) to the frontend entry point which switches the displayed component based upon the URL. This uses the same router that we use in the LMS frontend. This component also reads the JSON configuration and exposes it to child components via Preact context.
• Add a GroupFormHeader component which contains the "Settings" and "Members" links. These links trigger client-side transitions. This component is rendered by CreateEditGroupForm component if the group_members flag is enabled.
• Add a placeholder page for the "Members" view

Group edit view with new tab bar:

"Members" tab placeholder:

[robertknight]

A known issue is that clicking the "Members" link doesn't warn if there are unsaved changes in the group settings form. I will address this in a follow-up.

[robertknight]

What are you planning to do if the URL is manually visited while the feature flag is disabled?

The user will see the membership management page, the same as if the flag was enabled. That's fine, since the purpose of the flag is just to hide the entry point in the main groups form.