Closes #706 and closes #695.

## Main Goals
- Introduce new UI to manage subscription to Activity Summaries for each
class.
- Add more flexibility in task scheduling to enable scheduling Activity
Summary sending w/ LMS sync in the same container.

## DB Changes
- UserClassRole: Added new `last_summary_sent_at`,
`subscribed_to_summaries` columns
- Class: Added new `last_summary_sent_at` column

## Authorization Model Changes
- New relationship `can_receive_summaries` is set to Moderators only.
Admins shouldn't be receiving summaries for all groups they have access
to.

## Server Changes
- New GET, POST and DELETE `/class/{class_id}/summarize/subscription`
endpoint to look up, add or remove subscription to a group's Activity
Summaries for the current user.

## Scripts Changes
- New `run_dynamic_tasks` allows for running a Uvicorn server in the
background with multiple tasks scheduled.