Adding long awaited es_ES translation for FluxCP.

I had this one done for more than a year, but was awaiting a revision it never had. I just added new strings and am releasing it on its untested, unrevised state so use at your own risk.

I welcome any comments/suggestions/(constructive) criticism/issues but please avoid bashing it if you can come up with a better translation than mine, that's disrespectful towards this effort I've made on the first place so that Spanish-speaking servers can have their interface translated to Spanish.