RSS/Atom: Allow to set up different refresh times per feed

From my understanding one can set pollIntervalSeconds in global config.yml to define how often feeds are checked. I think it would be very handy to allow to set different refresh times per each feed.

Start by locating config.yml and the pollIntervalSeconds setting, then trace where RSS/Atom feeds are scheduled for polling. Done means each feed can have its own refresh interval while the global setting continues to work as the default; the issue does not name specific tests or files.

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