luci-base: switch admin category node to firstnode() action

luci-base: switch admin category node to firstnode() action

After this change, luci-base will render the first module or application
page installed on the system, instead of rendering a "Component not found"
message when the status category is unavailable.

This allows for single-purpose LuCI installations like e.g. luci-base with
luci-app-travelmate which only presents application specific views without
any of the standard system pages.

Signed-off-by: Jo-Philipp Wich <redacted>