Support for @role mentions pinging Matrix users

[axiopaladin]

Description

Using the m.mentions property of a message, it should be possible for the bridge to make messages that have inserted a @role (on the discord side of bridge) to "ping" matrix users that are part of that group.

Example

Consider the discord role @admins, which includes users Alice and Bob. Currently, the message content may look like this after bridging to matrix:

"content": {
    "body": "pinging @admins",
    "msgtype": "m.text",
    "format": "org.matrix.custom.html",
    "formatted_body": "pinging <font color=\"#123456\"><strong>@admins</strong></font>",
    }

Alice has the matrix account @alice:example.org and is logged into the bridge for double-puppeting. Bob is not logged into the bridge, so mautrix has created a pseudo-user for his messages with the username @discord_0123:example.org. When a bridged discord message includes a valid @admins ping, this feature request suggests that the bridge should append the m.mentions property to the message event payload, like so:

"content": {
    "body": "pinging @admins",
    "msgtype": "m.text",
    "format": "org.matrix.custom.html",
    "formatted_body": "pinging <font color=\"#123456\"><strong>@admins</strong></font>",
+   "m.mentions": {
+       "user_ids": ["@alice:example.org", "@discord_0123:example.org"]
+   }
}

(While the @discord_0123:example.org user doesn't have any need to be included in the mention, since it is an unmonitored account, it is shown here for clarity. In practice, it may be pruned to reduce message size.)