luci-theme-*: set mediaurlbase if it is the first time to be installed

luci-theme-*: set mediaurlbase if it is the first time to be installed

If we build multi-themes into firmware, each of them set itself
to be the default theme, what theme should it be?

To make it clear, we only set mediaurlbase if the theme is the
first time to be installed/built-in.

This resolve the issue that theme always change to somewhat default
after upgrading the firmware even with a config-keep-upgrade

Signed-off-by: Chen Minqiang <ptpt52@gmail.com>
(backported from commit fc150636b9393e31aa7d5a8d8320b111476ba2f1)