ENHANCEMENT: better pattern for dis-/enabling locale filter

This is an enhancement that allows a much easier pattern of disabling and enabling the locale filter.  It does not change any existing functionality, so is safe to merge at any point.

Once it is merged, the next step would be an audit of all places that are disabling the locale filter to make sure they are re-enabling it like they should.  That will be easier with this change.

That audit would catch bugs like silverstripe/silverstripe-translatable#126.  The pattern in this PR will make that change easier - less lines of code.

NOTE: that audit would also need to be done in framework and CMS, etc.