Add toast component in notifications

[gtsiolis]

Problem to solve

Notifications specs in #3398 included a toast component for visual feedback when a user changes their notification settings but wasn't included in #3513.

The toast component will be also useful in other pages like Integrations, see #3401.

User Experience goal

The user should be able to change notifications settings while having visual feedback upon change.

See also Don’t Prioritize Efficiency Over Expectations:

Users need to be continuously aware of the current status of the system. For users, seeing is believing: visual feedback must be displayed for every process a system undertakes.

Proposal

Let's a) implement a toast component and b) add it on notifications page.

Notifications

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gtsiolis]

Looping in @jldec since this could help us surface feedback or error messages to users in many places across the product.