Allow using middle mouse button to open a link in a new tab

[rudolfs]

A common web usage pattern is to open links for later reading in separate tabs by clicking the middle mouse button. This currently doesn't work, because we're only handling the on:click event. We could potentially use also the on:auxclick event, but unfortunately that doesn't work on Safari.

If we're considering switching out the router, we should also think about how we could enable this functionality.

One potential way to pull this off it is to only use anchor tags (<a href="#xxx>") and hash routing for navigation elements that are clickable. Then we wouldn't need to bother manually handling click events.

Here are a couple of areas in the app that I've tried middle-clicking out of habit and wondered why it didn't work: