PLT-2077 Support CJK hashtags (#4555)

* Add Korean character ranges to exist CJK pattern

* Add constant for CJK hashtags
Becuase most of keywords in CJK are two characters

* Add CJK ranges to hashtag pattern to handle it.

* Fixes hashtag pattern to apply numbers at last

* Remove a wrong test case
`test_` shouldn't be a hashtag

* Fix hashtag regex to support standard unicodes

* Remove wrong escapes from regex

webapp/tests/formatting_hashtags.test.jsx

@@ -73,12 +73,6 @@ describe('TextFormatting.Hashtags', function() {
             "<p><a class='mention-link' href='#' data-hashtag='#test'>#test</a>.</p>"
         );
 
-        // Known issue, trailing underscore is captured by the clientside regex but not the serverside one
-        assert.equal(
-            TextFormatting.formatText('#test_').trim(),
-            "<p><a class='mention-link' href='#' data-hashtag='#test_'>#test_</a></p>"
-        );
-
         assert.equal(
             TextFormatting.formatText('This is a sentence #test containing a hashtag').trim(),
             "<p>This is a sentence <a class='mention-link' href='#' data-hashtag='#test'>#test</a> containing a hashtag</p>"