feat: Add web app activity group empty states (M2-7339)

[farmerpaul]

📝 Description

🔗 Jira Ticket M2-7339

Add empty states to the activities list in the Web App.

Also fixed some minor spacing/styling surrounding activity list groups and the Restart button.

📸 Screenshots

No activities available:

Only scheduled activities available:

🪤 Peer Testing

1. Create an applet with only 1 activity whose auto-assign setting is turned off (so no assignments present).
2. Log into the Web App and open that applet.
   Expected outcome: The full-screen empty state should be shown
3. Create a new activity in that applet with auto-assign turned on, and schedule that activity to be available today sometime after the current time.
4. Log into the Web App and open that applet.
   Expected outcome: The "only scheduled activities available" empty state should be shown

[aws-amplify-us-east-1]

This pull request is automatically being deployed by Amplify Hosting (learn more).

Access this pull request here: https://pr-548.d15zn9do8xbzga.amplifyapp.com

[LashaunnaS]

Other than a few questions, this looks good.