[ZEPPELIN-4369] Redundant line separator for multiple text output

### What is this PR for?

There's redundant line separator when there's multiple text output. The root cause is that each `%text ` will append new line separator in frontend. This PR fix this issue by removing `%text ` is the previous output type is also `%text `.

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

### Todos
* [ ] - Task

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-4369

### How should this be tested?
* CI pass

### Screenshots (if appropriate)

Before
![before](https://user-images.githubusercontent.com/164491/66449195-c699db80-ea86-11e9-8065-a98d6842c916.gif)

After

![after](https://user-images.githubusercontent.com/164491/66449196-ca2d6280-ea86-11e9-8396-a12f6e5b9038.gif)

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

Author: Jeff Zhang <zjffdu@apache.org>

Closes #3478 from zjffdu/ZEPPELIN-4369 and squashes the following commits:

8c3d380db [Jeff Zhang] [ZEPPELIN-4369] Redundant line separator for multiple text output
2 files changed
tree: 040acffdf53ac5b1b54be234c0fa393bd0f336de
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  61. README.md
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  63. SECURITY-README.md
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README.md

Apache Zeppelin

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Zeppelin, a web-based notebook that enables interactive data analytics. You can make beautiful data-driven, interactive and collaborative documents with SQL, Scala and more.

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