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