Add more Korean translations and corrections in high standard (#2169)

* Add more Korean translation, corrections all reviewed by Korean community.

Contributed Steemians:
@asbear @bree1042 @chocolate1st @sjchoi @feyee95 @crowsaint @happyworkingmom @clayop @coffeenut @kmlee @sinner264 @seaturtle @yangmok701

* Add more Korean translation, corrections all reviewed by Korean community.

Fix 스팀 -> STEEM when it mentions the currency name.
Otherwise I will 스팀 as is because it is much better than "Steem" in Korean website. Google translator may tranlate 스팀 to Steam, but actually all the korean Steemians thinks 스팀 is Steem and we all agreed on it.

Contributed Steemians:
@asbear @bree1042 @chocolate1st @sjchoi @feyee95 @crowsaint @happyworkingmom @clayop @coffeenut @kmlee @sinner264 @seaturtle @yangmok701