Add mention property to DMChannel class

[Vioshim]

Summary

Implements DM Channel mentions as these have been properly reintroduced in the UI's update.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Sherazranjha]

Yy

[LeoCx1000]

Duplicte of #9382 - but also (in my opinion) making these mentionable feels pointless since only the DM's recipients can see the mention. It's not a global "take me to this user's DMs" button and could potentially cause confusion.