With the new group system for displaying ads we no longer can check if a
user belongs to a trust level group lower than specified. The other
problem is that ALL users including staff and higher trust levels all
belong to trust level 0. So without this fix if we say that an ad should
be visible to trust level 0 users then it will be shown to all users.
This fix adds a new default setting for each ad platform for excluding
trust level 3, 4, and staff users from being shown ads.
- Make display_groups hidden (they will be removed in a later commit)
- Switch to using only exclude_groups instead of display groups and
exclude groups
- rename showToDisplayGroups to showXAds for each provider
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