[ZEPPELIN-5266]: Enable extension of flexmark in markdown interpreter

### What is this PR for?
* Enable extension of flexmark in markdown interpreter

### What type of PR is it?
Improvement

### Todos
* [ ] - Make emoji auto-scale according to the size of text

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-5266

### How should this be tested?
* Create a cell of markdown type, set the interpreter to be flexmark and type in an emoji code, e.g. 😂

### Screenshots (if appropriate)
![image](https://user-images.githubusercontent.com/34905992/120780755-fef30500-c55a-11eb-9c58-a06c5e9f72d1.png)
![image](https://user-images.githubusercontent.com/34905992/120780771-01555f00-c55b-11eb-900c-aff350049818.png)

### Questions:
* None

Author: EricGao888 <[email protected]>

Closes apache#4130 from EricGao888/chufeng-fix-ZEPPELIN-5266 and squashes the following commits:

ecd432d [EricGao888] remove emoji images and use unicode instead
ba35fc8 [EricGao888] revert changes to karma.conf.js and package-lock.json
5ab45a3 [EricGao888] ZEPPELIN-5266: Enable extension of flexmark in markdown interpreter

markdown/src/main/java/org/apache/zeppelin/markdown/FlexmarkParser.java

@@ -19,6 +19,7 @@
 
 
 import com.vladsch.flexmark.ext.autolink.AutolinkExtension;
+import com.vladsch.flexmark.ext.emoji.EmojiExtension;
 import com.vladsch.flexmark.ext.gfm.strikethrough.StrikethroughExtension;
 import com.vladsch.flexmark.ext.tables.TablesExtension;
 import com.vladsch.flexmark.ext.typographic.TypographicExtension;
@@ -32,6 +33,8 @@
 
 import java.util.Arrays;
 
+import static com.vladsch.flexmark.ext.emoji.EmojiImageType.UNICODE_ONLY;
+
 /**
  * Flexmark Parser
  */
@@ -43,12 +46,14 @@ public class FlexmarkParser implements MarkdownParser {
   public FlexmarkParser() {
     MutableDataSet options = new MutableDataSet();
     options.set(Parser.EXTENSIONS, Arrays.asList(StrikethroughExtension.create(),
-        TablesExtension.create(),
-        UMLExtension.create(),
-        AutolinkExtension.create(),
-        WikiLinkExtension.create(),
-        TypographicExtension.create()));
+            TablesExtension.create(),
+            UMLExtension.create(),
+            AutolinkExtension.create(),
+            WikiLinkExtension.create(),
+            TypographicExtension.create(),
+            EmojiExtension.create()));
     options.set(HtmlRenderer.SOFT_BREAK, "<br />\n");
+    options.set(EmojiExtension.USE_IMAGE_TYPE, UNICODE_ONLY);
     parser = Parser.builder(options).build();
     renderer = HtmlRenderer.builder(options).build();
   }