This adds a new "description" field to the feed settings. This allows to
save custom description regarding a feed. It is also exported and
imported as "description" in OPML.
Go 1.22 introduced a new [for-range](https://go.dev/ref/spec#For_range)
construct that looks a tad better than the usual `for i := 0; i < N; i++`
construct. I also tool the liberty of replacing some
`for i := 0; i < len(myitemsarray); i++ { … myitemsarray[i] …}`
with `for item := range myitemsarray` when `myitemsarray` contains only pointers.
No need for a `BETWEEN`: we want to filter on entries published in the last
week, no need to express is as "entries published between now and last week",
"entries published after last week" is enough.
