Add design standard & implementation for alerts

[nwittwer]

There's currently no setup for customized alerts which will provide feedback to the user that something went wrong (preferably with specific next actions and details!). Because of this, there's still a lot of console.log() and alert() usage.

Examples of where alerts occur:

• Invalid URL entered in the URL bar (example: "my-url"). This currently just displays a default alert().
• If a URL returns error "X-Frame-Origin". Currently there's no UI alert, just an error in the console.
• Trying to delete all of the artboards on the screen. There must be at least 1.