Fixes #4432. This pull request has been made possible by the backing of my Patreon patrons.

The current copy-paste behavior is a mixture of the browser's default copy behavior, and the Draft.js paste processing.

With this Draftail update, we now override the defaults for both copy and paste between editors:

• Copy should fill the clipboard with the same markup as before, except with a serialised version of the Draft.js content included, and with less inline styles (can lead to some discrepancies in how word processors receive the pasted content)
• Paste should work the same in all scenarios, except when the content comes from a Draftail editor. There, it will insert the Draft.js content directly instead of its HTML representation.

Tested in:

• Windows 10, IE11 (no support, default "plain text" behavior)
• Windows 10, Edge 17
• macOS 10.13, Firefox latest
• macOS 10.13, Chrome latest
• macOS 10.13, Safari latest
• iOS 11.3 Safari
• Android Chrome 7.1

This fixes quite a few of the known issues with Draft.js (#4274), see the potential diff over at springload/draftail#147 (comment). I'll update the list myself once this PR is merged.

The underlying code changes are:

• Override Draft.js copy-paste to preserve full editor content thibaudcolas/draftjs-conductor#2 (new copy-paste processing)
• bug(copy-paste): preserve line breaks for pasting into word processors thibaudcolas/draftjs-conductor#3 (bug fix)
• Add Draft.js copy-paste handling overrides from draftjs-conductor. Fix #147 springload/draftail#155 (integration into the editor)