FIX Do not cache current site config

[emteknetnz]

Issue silverstripe/.github#272

When running behat on siteconfig from a recipe-kitchen-sink context only, the following behat tests will fail

• vendor/silverstripe/siteconfig/tests/behat/features/manage-page-permissions.feature:87
• vendor/silverstripe/siteconfig/tests/behat/features/manage-page-permissions.feature:95

The fail because $this->CanEditType in SiteConfig::canEditPages() will incorrectly by 'LoggedInUsers' instead of 'OnlyTheseUsers' - not this is NOT run-over from another behat tests, it happens when the tests are run individually

Beucase SiteConfig::current_site_config() is used at some point, and it will use a cached version of SiteConfig::get_one(), which is getting set at some point in recipe-kitchen-sink (I wasn't able to work out exactly where) which it normally isn't during installer.

I've had a look to see if SiteConfig::current_site_config() is being used in loops anywhere and it isn't, so the performance loss on not using the cache should be basically irrelevant

[GuySartorelli]

I don't like this change at all. Siteconfig is used all over the place in projects which is why it uses a singleton in the first place.

Have you tried putting a breakpoint somewhere like __set() or setField() to see when CanEditType gets set?
You can make the breakpoint conditional, e.g.

if ($fieldName === 'CanEditType') {
    echo ''; // has no effect but you can put a breakpoint here
}

[emteknetnz]

Replaced by silverstripe/silverstripe-cms#2959