Handle system-group names in rendering @-mentions

[gnprice]

This is a follow-up to:

• Handle user-group renames in rendering @-mentions #1259

When rendering an @-mention of a group that is a system group, the group won't appear in our user-groups data. With #1259 we'll therefore still fall back to the name that's in the HTML, which will be a display-name for the group in English.

Instead, when rendering a system group, we should use an appropriate UI string which we include in our strings for translation. For the display-names the web app uses for system groups, see this message.

Related issues:

• Handle user-group renames in rendering @-mentions #1259