Pasting into email composer doesn't convert line-breaks

[sefsh]

Description

Pasting into the email composer works to some degree, but some things aren't respected. Pasted content from some apps does get line-breaks respected but with Slack (web app or Electron web app) being one where it isn't there ought to be more.

Steps to reproduce

1. Go to Email composer
2. Copy a multi-line message from Slack
3. Paste into the composer

Expected Behaviour

Something resembling the original formatting and line-breaks:

Actual Behaviour

• Bold formatting is respected
• (Bullet points aren't but lists aren't supported yet anyway!)
• Line-breaks aren't respected.

[richardolsson]

This will hopefully be fixed by #2463, but more testing is needed.