Better mentions in Slack messages

Currently it's hard to mention Jira users (like reporter, assignee) in Slack messages since Slack requires the slack user id.

It should be possible to lookup a user's Slack ID on the fly:

https://api.slack.com/messaging/composing/formatting#mentioning-users

https://api.slack.com/methods/users.list

Requires some changes to our message editor though (ADF would help), so we can tell when a user is mentioned in a slack message, to look them up.

This also happen when you copy the comment body and send via external messages. If at mention users are on the comment body the accountId will show instead.