A. Converter update per HTML encoding spec along with changes in
  the encoding name alias table.
B. Remove all the codes for converters Blink and Chromium do not need
(SCSU, Lotus, ISO-2022-xx other than JP, BOCU, UTF-7, etc).

This is reapplying the following CLs (that we used for ICU 52.1) to ICU 54.1 :

https://codereview.chromium.org/598383002
https://codereview.chromium.org/654153002

We have two upstream bugs filed for A and B above:
  http://www.icu-project.org/trac/ticket/11296
  http://www.icu-project.org/trac/ticket/10303

In addiition to A and B, we unified Big5 and Big5-HKSCS per
the encoding spec (bug 277868). That also includes properly supporting
the four 2-character sequences ( see http://crbug.com/277868#c3 ).
big5_gen.sh deviates from the current spec to work around a bug
in the spec. (see https://www.w3.org/Bugs/Public/show_bug.cgi?id=27878)

Moreover, ucmlocal.mk is added to list only encodings we want to support.

Also, tighten the state table for windows-946-2000.ucm that we use
for EUC-KR for now. And, drop 'base' map for windows-{936,949}-2000.ucm.

Finally, add euc-kr-html.ucm along with scripts/euckr_gen.sh, but
it is not yet used pending the resolution of bug 450312.

Data size checkpoint: 20,566,864 bytes (the original ICU 54=25,343,024)

BUG=277868, 428145, 450312
TEST=net_unittests --gtest_filter="*ilenameUtil*"
TEST=base_unittests --gtest_filter="*Conv*"
TEST=browser_tests --gtest_filter="*ncoding*"
TEST=Blink: fast/encoding/*
R=jsbell@chromium.org, mark@chromium.org

Review URL: https://codereview.chromium.org/839713003