Ensure that the first time a page is opened, it's scanned as soon as possible

[matatk]

There's an optimisation in #273 for situations in which the user is moving between tabs rapidly, i.e. via the keyboard. In this case, a one-second timer starts when the page becomes visible, and a scan is only done if the page is still visible after that. This seems like a nice touch, but it has a downside: currently that same timer applies when a page is first loaded, and that is making the extension feel a bit slower compared to how it used to feel.