Implement notification email handler

.env.example

docker-compose.yml.example

package-lock.json

package.json

src/lib/notifications/channelTypes.js

src/lib/server/mail.js

src/lib/server/notifications/emailHandler.ts

src/lib/server/notifications/formatter.ts

src/lib/server/notifications/handler.ts

src/lib/server/notifications/webhookHandler.ts

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

src/routes/api/users.js