Implement SnackBar instead of Toast notifications

[sbrl]

This should be less intrusive for the user if done right.

Also, we want to avoid showing notifications when we load from the Internet - only when we load from the local cache or when something goes wrong....

https://developer.android.com/training/snackbar/showing

[sbrl]

Looks like Snackbars aren't working very well, since even with a CoordinatorLayout they:

• Sometimes don't show at all
• You can't swipe them away
• They are in the worst possible place and obscure the rest of the UI

Until we can find a solution for this, we can't use Snackbars really :-/