Don't use a slice of pointers to opml items, when we can simply use a slice of
items instead. This should reduce the amount of memory allocations and the
number of indirections the GC has to process, speedup up the import process.
Note that this doesn't introduce any additional copies, as the only time a
slice of subscription is created, the items are created and inserted inline.
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.
As per [OPML 2.0 specification]:
> Each sub-element of the body of the OPML document is a node of type rss or an outline element that contains nodes of type rss.
> Required attributes: type, text, xmlUrl.
[OPML 2.0 specification]: http://opml.org/spec2.opml#subscriptionLists
