[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