Handle user-group renames in rendering @-mentions

[gnprice]

Currently we render @-mentions of user groups using the group's name as recorded in the HTML. If the mentioned group has been renamed since the message was sent (or was last edited), this will be its old name. Instead, we should show the mentioned group's current name.

To do this we'll use the mentioned group's ID, which is also encoded in the HTML of the @-mention. Then we can look that up in our user-groups data, once we have that (#662). If the group is unknown, we can fall back to the name that's in the HTML.

This behavior is specified in docs just written today: https://zulip.com/api/message-formatting#mentions (or maybe this link; added in zulip/zulip@3aa2671), but I believe it's been the web app's behavior for a long time.

Related issues

• Prerequisite: Store user groups #662
• Similar features for other types of content node:
  Handle user renames in rendering @-mentions #1258
  Handle channel renames in rendering #-mentions #1047