Read from RDS

[MinhxNguyen7]

Summary

• Switch to read from RDS without changing the schema or deployment.
• Remove legacy MongoDB users. This will only affect users that have not saved a schedule in over a year.
• Still write to DDB without reading from it.
  • This allows us to revert this PR and retrieve all of the saved user data during the intervening period.

Testing

• The test plan was designed by looking at ScheduleSaveState schema and making sure that we test the read and write
  of every attribute.
• I have run through this, and everything looks good, but someone else should go through it as well as suggest more testing steps.

Test Plan

• Create a schedule and add a section to it.
• Create another schedule and add a different section as well as a custom event.
• Save the user data.
• Clear the site data and reload the page.
• Load the previously-saved user.
• Verify that the previously-added data populates, and the order of schedules is correct.
• Delete a schedule, and repeat the reloading and verification procedures.

Issues

• Closes Migrate to RDS #970

Future Follow-Up

• Remove DDB and vestigial scripts once the we're 100% sure that prod is good.

[adcockdalton]

Code looks clean. Checking on staging now.

[adcockdalton]

Test Plan

• Create a schedule and add a section to it.
• Create another schedule and add a different section as well as a custom event.
• Save the user data.
• Clear the site data and reload the page.
• Load the previously-saved user.
• Verify that the previously-added data populates, and the order of schedules is correct.
• Delete a schedule, and repeat the reloading and verification procedures.
  Additionally:
• Create an additional user and mimic the prior process
• Confirm that a 7-day custom event has intended behavior
• Ensure that all facets of data persist for custom events (color, map location)
• Ensure that "remember me" has intended functionality when using multiple users

Looks good to me. What an exciting PR! Cheers