Add feature notification signup modal

[kisamoto]

We want users to show interest in features. This helps us prioritise and them be informed when we add it.

Create a generic modal service that can be applied to any element that can pop up and inform the user that the feature is a work in progress and they can sign up to be modified about it.

Each feature has a unique ID and by signing up to a feature this adds their user info to a table in the background. They can also unsubscribe if they wish so the modal should be able to keep track of the features they are subscribed to (i.e. lazy load from the backend into a local state).