Open UnTab when content script isn't set

[blenderskool]

Some screens like Browser new tab, settings and few others cannot have a content script embedded in them. This makes it impossible to open UnTab and requires the user to switch to a tab with some webpage open.

Since UnTab already has index.html as a web-accessible resource which gets loaded in an iframe via the content script, for these special pages, we can directly open the URL itself in a new tab.