Make group search more flexible.

[dcavins]

While working on #6431, I realized that the group search could be more flexible, like the WP_User_Query search that allows the specification of where the wildcard should be set and which columns should be searched.

I'm attaching a patch that mostly duplicates the logic of WP_User_Query. The one difference is that our search has always assumed that the terms would be wild carded on both ends, so the default if no wildcard characters are detected is to follow the old behavior.

(Re #6431, then get_by_letter() could just use get().)

Add search_columns parameter and calculate wildcard placement in search string.

Yeah, this looks like a good change. Shared code might be worth a BP_Core_Search_Utils class or something in the long run, but not necessary for this ticket.

In 11383:

BP_Groups_Group::get(): Add search_column parameter.

Allow developers to specify which column should be searched and where
wildcard characters should be placed in BP_Groups_Group::get().

Fixes #7418.