[ZEPPELIN-1085] Make dropdown menu scrollbar always visible

[AhyoungRyu]

What is this PR for?

In Zeppelin docs website, there are 4 dropdown menus(Quick Start, Interpreters, Display System and More) in the navbar. Especially Interpreters and More menu have lots of lists so I made this dropdown menu scrollable in #1004.
Originally the scroll bar is shown up when a user is trying to scroll down. That's why it's hard to notice there are more menus in the dropdown menu. See this issue.

So I made this scroll bar always visible. But only for chrome & safari. Unfortunately, Firefox doesn't support custom scroll bar. See here.
In short, it says

There is no way to actually "force" Firefox render the old-style scrollbar since the default scrollbar used in the system is predefined by the OS itself (note that you can modify which scrollbar you want in System Preferences).

What type of PR is it?

Improvement

TODO

• : test this in major browsers

What is the Jira issue?

ZEPPELIN-1085

How should this be tested?

Build the Zeppelin docs website in your local and check the dropdown menu in navbar. The scrollbae has to be shown right after you click Interpreters or More menu.

Screenshots (if appropriate)

• Before : scrollbar is only shown up when you are trying to scroll up & down
  
• After : always visible
  
• In case of IE, you can always see the scrollbar. So it's not a problem originally.
  

Questions:

• Does the licenses files need update? no
• Is there breaking changes for older versions? no
• Does this needs documentation? no

[AhyoungRyu]

@jeffsteinmetz It will be appreciate if you can review this PR as a reporter of this issue :)

[Leemoonsoo]

Thanks @AhyoungRyu for improvement.
Tested with Chrome and Safari. LGTM

[prabhjyotsingh]

LGTM

[jeffsteinmetz]

LGTM
At least in Firefox, the scroll bar now displays for a few seconds (doesn't stay, but its an improvement).
Chrome and Safari work as expected.

Tested with:

• Chrome on OSX: Version 51.0.2704.103 (64-bit)
• Safari: Version 9.1.1 (10601.6.17)
• Firefox on OSX: 47.0.1

[Leemoonsoo]

Merge it into master if there is no more discussion

[AhyoungRyu]

@jeffsteinmetz Right. So Firefox users can know they can scroll down and there is more menus even though this PR couldn't be applied to Firefox.
Thanks for testing this @Leemoonsoo @jeffsteinmetz :)