Fix bug with highlighting text & drag region on macOS

[alexbrazier]

@RocketChat/core

Closes #410

Fixes bug with selecting text by changing drag region show in images below.
Green means it is draggable (meaning you can't highlight text). If you remove the server list it just gets rid of that drag region, and the rest stays the same.

[Nosferous]

Another part of this issue is that if you have a scroll bar due to many channels/DMs/Etc. the scroll bar is not clickable as a scroll bar. This means you can't scroll if you have a standard 2-button mouse without scroll-wheel/scroll-touch funtionality.

See screen:
I have a grey scroll bar on the left. If I click and try to scroll it just drags the Rocket.Chat.Electron window around.

Using Rocket.Chat.Electron: 2.6.1
Server: 0.54.2 [running onto Ubuntu 16.04 LTS]
My Client Machine: OSX 10.9.5

Sidenote- My Rocket.Chat.Electron app doesn't look exactly like the one in my screenshot. When using the app there is no grey title bar for the app window. But once I screenshot it- OSX is revealing it in the snap. (Just incase anyone questions if I'm actually on 2.6.1- I am. But OSX on 10.9.5 is revealing the title bar during built in screen shot process).

[rodrigok]

What about use just the top area to drag?

[alexbrazier]

@rodrigok I'm happy to change it to just the top bar. Hopefully it should be obvious enough for most people.