Plan for change to username formatting

[aoberoi]

Description

From the platform changelog:

The undocumented approach to mentioning users via the API — <@username> — will no longer function after September 12, 2018. Please reference with the user ID format (<@u123>) instead.

This undocumented approach is never explicitly depended on in this adapter, but its quite common for users of it to send messages that contain user mentions. We should try to get ahead of this one by offering everything a script needs to responsibly off-board from the deprecated behavior. here are some ideas:

• log a warning when this formatting is detected in a message, point to a description of how to do this correctly.
• attempt to replace these mentions with a properly formatted mention by querying the data in the Brain
• make it as easy as possible to structure a mention in a message (we've laid some groundwork with the SlackMention object attached to all incoming messages, how can we repurpose that?). add this usage to the docs so that it can be referenced in the warning.

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

[github-actions]

👋 It looks like this issue has been open for 30 days with no activity. We'll mark this as stale for now, and wait 10 days for an update or for further comment before closing this issue out.