Add alternative frontend redirection page

[DarkCat09]

Useful in iframes. For example, do not load YouTube embed, but load a txtdot page instead, where "Open in Invidious" / "Original page" can be chosen. Alternative frontends' URLs are configured in settings, #99

[DarkCat09]

@artegoser, we'll add this page as a new engine?

[DarkCat09]

Just realized that in the engine output HTML, we can set classes to apply txtdot's CSS.
So, yeah, I think the "alt frontend redirection page" should be added as a new engine, with txtdot-styled buttons in its output.

Upd: there will be "Home" and "Original page" buttons at the top in this case.
So maybe not:
"Redirect to alt frontend" / "Original page" buttons in a row,
but:
"Home" / "Original page" at the top
URL field (filled with URL configured in settings) / "Redirect" button.

Or keep it simple and create a separate txtdot route.