Slack: Add ability to mention user by username/email

The slack API allows mentioning users via their userID like so:
```
Hey <@U024BE7LH>, thanks for submitting your report.
```

However, almost any use-case for Notifications won't include the actul slack userID; Instead, it will have things like an email associated with a git commit.

The slack documentation (linked below) recommends using the `users.list` api to retrieve the ID via some other unique piece of information (such as an email).

Can we expand the engine to parse for things formatted as something like `{{.slack.userEmail}}`, and do a lookup?

It would be equally helpful if we could do the same for:
- channels: `Why not join <#C123ABC456>?`
- groups: `Hey , there's a new task in your queue.`

Reference:
- https://api.slack.com/reference/surfaces/formatting#mentioning-users
- https://api.slack.com/reference/surfaces/formatting#linking-channels

Start with the Slack formatting references in the issue and trace how notification templates are parsed and sent to Slack. Determine how user, channel, and group identifiers would be resolved from usernames or email addresses, then verify that the resulting mentions work for all three requested forms.

Written by the indexing model from the issue text.