Fix double loading on install

This issue arises because we're not checking the status of the tab before trying to inject. So when during installation a new tab is created and the extension activated this triggers the change from 'inactive' -> 'active' in TabState which embeds the sidebar into the loading tab. Later the update function is called when the tab loading is complete injecting the sidebar a second time.

We now explicitly check the tab status in the injection handler to prevent injection into a loading tab.

Fixes #1756.