Add support to take a screen capture and paste as a file upload to a Mattermost message draft

[jasonblais]

Add support to take a screen capture and paste it as a file upload to a Mattermost message draft.

Use case: Those not familiar with keyboard shortcuts and slash commands use UI elements to perform actions like screen captures. This is also natively supported in Jabber (screenshot below).

As a starting point, the proposal is for the plugin to capture the OS Paste event when you are adding a new drawing, and paste the clipboard image in the canvas.

In the future, we can additional capabilities such as using WebRTC to capture the desktop outside of the Mattermost application, as well as draw on the screen capture.

[jasonblais]

@jespino please review, make adjustments as needed and add a help wanted label?

I created this issue based on the conversation here https://community.mattermost.com/core/pl/c3ec8951qpd6inzjzuc6yda5aa