Prevent word breaking when wrapping discussions messages (fix #1820)

[noirbizarre]

This PR change the discussions word wrapping rule and prevent word to be breaked:

• layout has been a little bit refactored so all flexbox constraints apply (width in particular)
• scoped style attribute has been removed allowing style to apply on Markdown
• word-wrap and word-break are back to normal in the message body
• set word-wrap: break-word and word-break: break-all on message body anchors
• make pre > code scroll horizontal in case of overflow

Desktop

Mobile

Notes

Fixes #1820
Fixes #1747