[ZEPPELIN-1723] Math formula support library path error
[ZEPPELIN-1723] Math formula support library path error

### What is this PR for?
When user set `ZEPPELIN_SERVER_CONTEXT_PATH`, web cannot find MathJax library because root location is set to `/`.

### What type of PR is it?
Bug Fix

### What is the Jira issue?
[ZEPPELIN-1723](https://issues.apache.org/jira/browse/ZEPPELIN-1723)

### How should this be tested?
1. Checkout remote branch, and cherry-pick one commit from #1863 which is needed to try out this PR:
    ```
    $ ./dev/test_zeppelin_pr.py 1865
    $ git cherry-pick dbae64d
    $ mvn clean package -DskipTests -pl '!zeppelin-distribution,!alluxio,!ignite,!lens,!kylin,!scio,!hbase,!pig,!file,!flink,!cassandra,!elasticsearch,!bigquery,!postgresql,!jdbc,!python,!angular,!sh,!livy'
    ```
2. Set `ZEPPELIN_SERVER_CONTEXT_PATH` in conf/zeppelin-env.sh and start Zeppelin
3. See if error occurs in browser inspection console.
4. Test if you can use Math function with pegdown md interpreter:
   ```
   %md
   When \\(a \\ne 0\\), there are two solutions to \\(ax^2 + bx + c = 0\\) and they are
   $$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$
   ```
### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: Mina Lee <minalee@apache.org>

Closes #1865 from minahlee/ZEPPELIN-1723 and squashes the following commits:

2b06b21 [Mina Lee] Fix mathjax loading in dev mode
ccf4494 [Mina Lee] Add MathJax root location config

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