Change default bindings for Up/Down in command mode

Those now look at the history again.
Looking at the behavior in different applications:

- vim: History
- spacemacs: Completion if open, else history
- luakit: Completion if open, else history
- dwb: Always completion (has no history?)
- vimb: Nothing if completion open, else history
- vimperator: Always history

So this is consistent with at least some of them - the much more important
factor is that <Tab> is probably intuitively easy to discover if up/down doesn't
do what's expected, but <ctrl-p>/<ctrl-n> are not.

doc/help/settings.asciidoc

@@ -400,13 +400,13 @@ Default:
* +pass:[<Ctrl-U>]+: +pass:[rl-unix-line-discard]+
* +pass:[<Ctrl-W>]+: +pass:[rl-unix-word-rubout]+
* +pass:[<Ctrl-Y>]+: +pass:[rl-yank]+
* +pass:[<Down>]+: +pass:[completion-item-focus next]+
* +pass:[<Down>]+: +pass:[command-history-next]+
* +pass:[<Escape>]+: +pass:[leave-mode]+
* +pass:[<Return>]+: +pass:[command-accept]+
* +pass:[<Shift-Delete>]+: +pass:[completion-item-del]+
* +pass:[<Shift-Tab>]+: +pass:[completion-item-focus prev]+
* +pass:[<Tab>]+: +pass:[completion-item-focus next]+
* +pass:[<Up>]+: +pass:[completion-item-focus prev]+
* +pass:[<Up>]+: +pass:[command-history-prev]+
- +pass:[hint]+:

* +pass:[<Ctrl-B>]+: +pass:[hint all tab-bg]+

qutebrowser/config/configdata.yml

@@ -2026,13 +2026,12 @@ bindings.default:
    passthrough:
      <Ctrl-V>: leave-mode
    command:
      # FIXME:conf what to do about up/down?
      <Ctrl-P>: command-history-prev
      <Ctrl-N>: command-history-next
      <Up>: command-history-prev
      <Down>: command-history-next
      <Shift-Tab>: completion-item-focus prev
      <Up>: completion-item-focus prev
      <Tab>: completion-item-focus next
      <Down>: completion-item-focus next
      <Ctrl-Tab>: completion-item-focus next-category
      <Ctrl-Shift-Tab>: completion-item-focus prev-category
      <Ctrl-D>: completion-item-del