Add admin interface for managing notifications

src/routes/admin/api/[slug]/+server.js

src/routes/admin/api/notificationChannels.js

src/routes/admin/api/users.js

src/routes/admin/user/[id]/+page.server.js

src/routes/admin/user/[id]/+page.svelte

src/routes/admin/user/[id]/NotificationChannelItem.svelte

src/routes/admin/user/[id]/NotificationChannelList.svelte