Use derived context for WeeklyRoundup background task

[dvdchr]

Fixes #17639

This attempts to fix a crash due to the managed object context being accessed from the wrong thread. The WeeklyRoundupDataProvider is now initialized with a derived context so all of its operations won't be running on the main context and hopefully prevents overlap with any operations running in the main queue.

To test:

Refer to the steps described in #17066 and #17116, and make sure everything works.

Regression Notes

1. Potential unintended areas of impact
   The Weekly Roundup feature might not work as expected.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested the scenarios described in Local Weekly Roundup #17066 and Implement weekly roundup toggle #17116.

3. What automated tests I added (or what prevented me from doing so)
   n/a.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[diegoreymendez]

The failing test is due to an unrelated issue with a timeout that's too aggressive.

[mokagio]

@dvdchr this has been bundled as part of 18.8 beta 4 (18.8.0.4), which is currently building in CI.

Thanks for your work 🙌