Make the message body sync window configurable (currently hardcoded to 3 months)

Feature Suggestions
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1

Summary

Please add a preference in the UI to control how far back Mailspring downloads and indexes message bodies. The window is seems currently hardcoded to three months in the sync engine, and the code comment suggests a preference was always the intention:

time_t SyncWorker::maxAgeForBodySync(Folder & folder) {
    return 24 * 60 * 60 * 30 * 3; // three months TODO pref!
}

(Mailspring-Sync, SyncWorker.cpp)