When you have a lot of groups that share a common prefix (e.g. Team_...) it is
useful to be able to autocomplete group names without having to type the common
prefix.

Add a webapi feature that needs to be enabled for the extended search to be
enabled. This to avoid problems on installation where enabling contains
searches may affect performance too much.

Add a new parameter `contains` to /api/groups/ that, when set, changes the
filtering from "istartswith" to "icontains" and thus matches the key anywhere
in the name (or display name if displayname=1).

Change the review request editor view to set this contains=1 in the request and
matchContains option on the autocomplete widget to make it all work.

New unit tests passes.
Manual tested that it is possible to autocomplete names without typing the prefix.
Documentation looks ok.