Abstract permissions base from default deny and allow

[chiatt]

In version 7.6, there are now two permissions frameworks in core arches, 'standard' and 'default_deny'. The problem with the name 'standard' is that it does not clearly indicate that it is 'default allow'. In v8 we may want to change the permissions framework that arches ships with to default deny, so we wouldn't want default allow to be called 'standard'. Also, it would be clearer to future users and developers if the name were more explicit.

As @chrabyrd proposes here, we could do this by creating a 'PermissionsBase' class from which 'DefaultDeny' and 'DefaultAllow' would inherit.

Additionally, I'd suggest we remove the prefix 'Arches' from the names of both as it's really not necessary.