Improve order of mention autocomplete results

What does this MR do and why?

Orders project members ahead of groups since these are more commonly used. Also orders groups by full_path instead of just path.

Screenshots or screen recordings

This is how it works today:

Project members aren't shown in the initial autocomplete if you have a lot of groups because we only show 10 results. Also, notice that the groups are ordered by path rather than full path.

The ordering is also kept when searching:

Notice that the groups are ahead of project members.

This MR switches the ordering so that project members are ahead of groups and groups are ordered by full_path.

MR acceptance checklist

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