Guard against corrupted settings

[philippotto]

There are multiple ways to do this:

• enforce that the layer settings match our schema (e.g., if a layer opacity is none, change it to the default layer opacity)
• catch react errors in the settings component and reset all settings to default

The case which we observed twice now is that the layer alpha value was set to null for some reason, causing wk to crash as soon as the settings sidebar was opened.

[philippotto]

I think this is covered now by https://github.com/scalableminds/webknossos/pull/4845/files