allow to enable/disable notifications per-account

[adbenitez]

close #3126

[adbenitez]

currently ui.muted config is used, we need a proper core config for this

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[r10s]

i created a PR offering is_muted at https://github.com/deltachat/deltachat-core-rust/pull/5759

btw, i think we should migrate the old setting, so that we do not start adding notifications the user has disabled before. we could do that similar to "migrating chat backgrounds" (which can be removed then)

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