Remove not remembering layer

[jcklie]

Is your feature request related to a problem? Please describe. Using !remember layer is painful, so why not remove it.

Describe the solution you'd like

• [x] Hide "remember layer" option behind a feature flag (merged for 0.20.0)
• [ ] Remove !remember layer code
• [ ] Add a label to layer dropdown so that it is clear that it is the layer to create NEW annotations, not the layer of the currently selected annotation

[reckart]

Before finally removing it, we should hide it behind a feature flag for a while - adding it back would be really annoying.