Support importing and exporting event logs from HistoryServer web portal

Many users who met the problems when running Spark applications will send the logs to some experienced guys to seek help, normally this running log is too verbose to dig into the details.

This proposal is trying to handle this problem: user could download the event log of a particular application, and send to other guys, other guys could replay this log by uploading to the history server, it's quite useful to debug and find issue for many customers who are using Spark.

Here is the screen shot, you could download and upload event log through button click, the imported event log will be named with suffix "-imported".