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.

[mokagio]

@dvdchr how do you feel about targeting the release branch for this?

The metrics for the crash look encouraging in that the impact is limited, but, because of the December release pause, 18.9 is almost a month away from reaching the users.

[dvdchr]

how do you feel about targeting the release branch for this?

The metrics for the crash look encouraging in that the impact is limited, but, because of the December release pause, 18.9 is almost a month away from reaching the users.

@mokagio Yeah, that makes sense! As long as @diegoreymendez feels confident with the fix. Thank you for the heads up! I'll create a new PR based on release/18.8, cherry-picking the commit. 🙂

[peril-wordpress-mobile]

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

[dvdchr]

Closing in favor of #17642.