Make form submissions open in a new tab and leave Via

When a user clicks on a link in Via it opens the link target in a new tab that is _not_ proxied by Via: https://github.com/hypothesis/viahtml/pull/9

But when the navigation is triggered by a form submission this still navigates the original tab and does not leave Via. For example Google and Bing's search forms do this kind of navigation.

We need to intercept these form submissions and make them open in a new, non-proxied tab like normal links do.

A JavaScript one-liner for programmatic link clicks was given in this thread, and it was suggested that something similar could work for form submissions:

https://hypothes-is.slack.com/archives/C1M8NH76X/p1621418996025700