luci-app-shadowsocks-libev: move plugin options to server sections

In this change, plugin options are moved from component settings to
only server settings.  This should make the configuration easier.  And
it will be less error-prone as it's impossible now for users to
inconsisitent plugin settings for instances referring to the same remote
server

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>