[NEW] Add toolbar buttons for iframe API

[sampaiodiego]

@RocketChat/core

Closes #7433

This PR adds two new external commands: set-toolbar-button and remove-toolbar-button.

To add/set a button at the top toolbar:

document.querySelector('iframe').contentWindow.postMessage({
  externalCommand: 'set-toolbar-button',
  id: 'resize',
  icon: 'icon-resize-small'
}, '*')

A button can be updated using the same id:

document.querySelector('iframe').contentWindow.postMessage({
  externalCommand: 'set-toolbar-button',
  id: 'resize',
  icon: 'icon-resize-full'
}, '*')

To remove a toolbar button:

document.querySelector('iframe').contentWindow.postMessage({
  externalCommand: 'remove-toolbar-button',
  id: 'resize'
}, '*')

When a button is clicked, the parent window will receive a postMessage called click-toolbar-button and an object with button's id

[gdelavald]

Having multiple buttons can end up hiding the channel name in some cases (for smaller screens), but I think that's the viable solution anyway.