Set system to read only mode

Sometimes when doing upgrades etc it would be good to have the system in a read only state

You could guarantee this by creating a read-only database user, then changing it in ```/etc/variantgird/settings_config.json```

This would likely throw heaps of errors - so a way to test this would be to make that user and see what dies

We could wrap everything (eg eventlog etc) behind ```settings.READ_ONLY_MODE``` and disable all POST/PUT