Block access is checked even if the block's region is never rendered

I just noticed that BlockRepository::getVisibleBlocksPerRegion() checks the access on every block from every region and then only keeps blocks from regions returned by ActiveTheme::getRegions(), so some of these access checks are called for nothing.

I opened a MR that makes it check only blocks from regions returned by ActiveTheme::getRegions().
This might fix 🐛 Block access is checked even if block is inside a group that is not displayed Active but not the issue with theme regions that are not rendered.