[SPARK-8862][SQL]Support multiple SQLContexts in Web UI

This is a follow-up PR to solve the UI issue when there are multiple SQLContexts. Each SQLContext has a separate tab and contains queries which are executed by this SQLContext.

<img width="1366" alt="multiple sqlcontexts" src="https://cloud.githubusercontent.com/assets/1000778/9088391/54584434-3bc2-11e5-9caf-94c2b0da528e.png">

Author: zsxwing <zsxwing@gmail.com>

Closes #7962 from zsxwing/multi-sqlcontext-ui and squashes the following commits:

cf661e1 [zsxwing] sql -> SQL
39b0c97 [zsxwing] Support multiple SQLContexts in Web UI