RSS: Fix bad usage of safe_substitute.
RSS: Honor supybot.plugins.RSS.initialAnnounceHeadlines.
RSS: Fix typo.
src: update help text of commands.nested.brackets

The shift part only applied to people on American keyboard and not all
users are on that, so it only caused confusion.

Strong historical motivations should be enough for most of people as I
wasn't able to remember the correct brackets when I tried changing them
to `<>` which I feel are the easiest to type on Finnish/Swedish keyboard.

Also update messages.pot & l10n-fi.
Allow --disable-multiprocessing in main supybot script.
Move disableMultiprocessing from supybot.conf to supybot.world in order for it not to be in the config file.
commands: Explicitely close the Queue object.
scripts/supybot: Fix typo.
Fix help text of supybot.commands.nested.
Admin: Update .po files
Admin: Clarify error on 435 numeric
Tell in the README that we support Pypy3.