Add popup for warning messages from the API

[overflew]

E.g.:

Voting rules:

• can't vote more than once
• can't vote for own song

Queuing rules

• Song already queued
• Limit of 5 queued songs reached

May as well use Material's Snackbar?
https://material.angular.io/components/snack-bar/overview

[SkyeBelinda]

I'll check this out 8)