[SPARK-2017] [UI] Stage page hangs with many tasks

(This reopens a patch that was closed in the past: #6248)

When you view the stage page while running the following:
```
sc.parallelize(1 to X, 10000).count()
```
The page never loads, the job is stalled, and you end up running into an OOM:
```
HTTP ERROR 500

Problem accessing /stages/stage/. Reason:
    Server Error
Caused by:
java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOf(Arrays.java:2367)
    at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:130)
```
This patch compresses Jetty responses in gzip. The correct long-term fix is to add pagination.

Author: Andrew Or <andrew@databricks.com>

Closes #7296 from andrewor14/gzip-jetty and squashes the following commits:

a051c64 [Andrew Or] Use GZIP to compress Jetty responses

core/src/main/scala/org/apache/spark/ui/JettyUtils.scala

@@ -210,10 +210,16 @@ private[spark] object JettyUtils extends Logging {
       conf: SparkConf,
       serverName: String = ""): ServerInfo = {
 
-    val collection = new ContextHandlerCollection
-    collection.setHandlers(handlers.toArray)
     addFilters(handlers, conf)
 
+    val collection = new ContextHandlerCollection
+    val gzipHandlers = handlers.map { h =>
+      val gzipHandler = new GzipHandler
+      gzipHandler.setHandler(h)
+      gzipHandler
+    }
+    collection.setHandlers(gzipHandlers.toArray)
+
     // Bind to the given port, or throw a java.net.BindException if the port is occupied
     def connect(currentPort: Int): (Server, Int) = {
       val server = new Server(new InetSocketAddress(hostName, currentPort))