"New!" customizable toast message

Add a toast showed only one time per user - that fades after 6 seconds and with a X to close - to communicate something of our choosing. This needs to be as easy as possible to customize (change text) and to turn it off.

It will be used to communicate different things. For example, after launching MyStore, we could use this to let users know that they can go to the settings section and customize it (links should be permitted so we can redirect the user if we want). This is to encourage users to be aware and make use of the new features that we release.