[ux] Add a search box to every page

[JanKoehnlein]

I think people mostly come to the open VSX page to search for extensions. On the page for an extension, there is no way to start another search. So if that's not the extension you were looking for, the only way to achieve this is by either using the back button of the browser or clicking on the Open VSX logo on the top left. I don't think that's very suggestive nor comfortable.

I'd prefer to have a search box at least on the individual extension pages, such that I can directly continue with my research for extensions.