Problem: Error messages look bad

This MR addresses some of this user feedback:

• "This looks really scary to me? Is this a log?"
• "Maybe put it in message box, or in a console log?"

(Reference: https://gitlab.com/exosphere/exosphere/wikis/user-testing/person-a#2018-11-10-round-2)

Solution: Added pablen/toasty package and use it to show errors. It's easy to dismiss the toasts, and there won't be duplicates (because we use Toasty.addToastIfUnique).

Also added a dedicated 'Message Log' menu item and view. This is where the user can see all the old messages, even the dismissed ones.

This is what it looks like now:

Fixes #123 (closed) ('Better error message display / reintroduce something like toast')

Coincidentally also fixes #147 (closed) ('Image tiles not tiling horizontally')