[#982] Background syncing

[LukasKorba]

Closes #982
Closes #984
Closes #987

• Background processing task implemented
• Doc for the Background Synchronization added
• changelog update
• custom wifi check

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[pacu]

Congrats on this long-awaited feature ❤️🎉

[LukasKorba]

I realized we should implement at least one more advanced thing for background syncing - I call it Advanced Background Syncing, see #987. The scheduling in background tasks API lacks recurrence and typically this is solved with another background task that's only purpose is to create the recurrence "manually". Such task doesn't require a network or power and can finish its job real quick. With this extra work we increased the efficiency and meaning of background syncing by a huge leap.

I updated the code.

[HonzaR]

I mainly reviewed the documentation and left some advice on it.

I have one more general note on the background syncing feature, which could be addressed in a follow-up. Should we provide a straightforward UI for tweaking when the BG sync is triggered? So we ensure this feature is available for everyone because having it restricted on time, power, and connection type could make most of the users asking why the feature is not working. The new settings could have:

• on/off for the whole sync feature (default on)
• required plugged-in power (default on)
• required wifi connection (default on)
• time picker (default 3 am)

[HonzaR]

I recommend using reset numbering here as it's more future-proof, i.e.

1. Add a capability of `background modes` in the settings of the xcode project.
1. Turn the `Background Processing` mode on in the new capability.
...

See https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#lists