Configurable wildcard search types in BP_User_Query

[DJPaul]

For #3278, I need to be able to set the wildcards used around the search_terms argument in BP_User_Query.

Right now, we add % to both sides of the search term. For auto-suggestions, I need to be able to do a member search in BP_User_Query but only allowing the wildcard on the right-hand side of the search string, e.g. `hello%' instead of '%hello%'.

Attached patch adds a new search_wildcard argument to BP_User_Query, with a unit test.

[DJPaul]

I'm reasonably happy with this approach, but wanted opinions from other devs, particularly Boone, before going ahead.

[boonebgorges]

This approach looks pretty good to me. Thanks, DJPaul.

[djpaul]

In 8675:

Core: add new search_wildcard parameter to BP_User_Query.

This new parameter controls where BuddyPress places MySQL wildcard characters around the search term (when it's set).
Prior to this change, BuddyPress added a wildcard character to both ends of the search term, but for some use cases, a more restrictive search pattern is required; you can specify "left", "right", or "both" (the default) to pick where the wildcard is added.

Fixes #5769